(for those of you that don't want to read a few short carbidizing creates very hard microserrations that greatly aid in slicing, but restrict pushcuts somewhat. I'm really enjoying my carbidized personal knives. Only sharpen the non-carbidized side of the blade)

Carbidizing is the process of adhering carbides to a metal substrate. The 2 most popular carbides to use are tungsten carbide, and titanium carbide. Both are very hard materials, that are manufactured into drill bits, lathe tools, burrs, etc that are made for cutting steel (even hardened steel), stone, etc. They are very abrasion resistant.
You can see why carbides are preferred, but you may be asking yourself why not just use pure carbide instead of adhering it to another metal? The reason is 2 fold....#1 is cost; carbides are very pricey, the #2 reason is carbides are also fragile. If you were to make a circular saw blade from solid carbide instead of brazing just carbide tips on, the blade would cost a small fortune and could potentially shatter. With knives a solid carbide blade would easily break from lateral stress or chopping hard materials.
So by welding a thin layer of carbide onto a softer, stronger metal, you not only end up with a product thats more economical but also stronger.
Most industries braze a thick carbide onto the products cutting edge..for example the afore mentioned circular saw blades with carbide teeth, carbide lathe tools, or carbide tipped masonry drill bits. While this works very well for those products, a knife blade has different demands. Knife edges need to be thin enough to cut well, but thin means fragile since there is less material there to offer strength. Carbide tools are typically ground thick, with fairly wide angles...something that won't work well for most knives.
In the knife industry carbidizing is done by microwelding very thin, small diameter puddles of carbide onto the steel or titanium. Basically a knife is made the same way it traditionally is, but at the end of the process the carbide is microwelded onto one side of the cutting edge. Why only one side instead of both? Because even though the carbide makes the cutting edge last longer, it will of course need to be sharpened from time to time and the first time you sharpen the knife it wears away the carbide. So what knifemakers do is coat only one side of the blade, then have the blades edge bevel only ground on one side. Then each time you sharpen the knife you only sharpen the non carbidized edge. That way the carbides are on there for the life of the knife.

So what are the benefits and drawbacks?
The carbides welded in place have a hardness of 70-80rc. Most custom knives have an rc around 60 (most cheap factory knives have an rc around 50-55, so you can see what a jump going from 60rc to 70+ is). This makes the edge more abrasion resistant, and the knife will cut longer between sharpening.
Since the carbide layer is very thin, measured between .001-.007” depending on the application, the knife retains all its original strength, while being able to cut longer.
Since the small puddles of welded carbide are not perfectly smooth, when the knife is sharpened it forms micro serrations, which act like a saw blade, allowing very nice slicing action.
Can be applied to titanium to greatly enhance edge holding.
Since microserrations are formed, the edge isn't as smooth as a typical knife and it will not get as sharp. On some blades it can get shaving sharp, but it will never get to be that super-duper-ultra-hair-popping sharp.
The carbidized edge is textured, almost like a sand blast finish, or a nail file. In most cases this is no big deal, but I thought I would mention it.
Added cost. Its another step in the process and time/materials=added cost.
The blade will not do push-cuts as well as a noncarbidized edge thats super scary sharp.
Should every knife have a carbidized edge/is it right for everyone?
Since carbidized edges form microserrations a carbidized knife will out slice the same knife without a carbidized edge. Thats where carbidizing excels.

But since the knife is not able to get super-scary sharp it will not push cut (cutting an object without slicing/sliding the edge on it) as well. So some blades are not meant for it...for example kiridashi with a steep/more obtuse angle where slicing motions are not easy to make.
Who should buy carbidizing, and who should avoid it?
Buy it if you want a very long lasting edge intended for slicing.
Buy it if you want to go longer between sharpening.
Don't buy it if you plan to mostly push cut with the knife.
Don't buy it if you enjoy spending a lot of time sharpening.(for some of us sitting in front of a sharpening stone is almost zen-like, for others its just a way to get the knife ready to use)
Don't buy it if you want an edge to be as sharp as possible.
Credit where credit is due.
I first had the idea of applying carbide to knives about 10yrs ago when I used my first carbide tool and thought “this kicks butt!” Over the next few years I spent some time looking into how to apply carbide. Brazing, hardfacing, etc. While some looked like they would work, each method had drawbacks. Then I came across Warren Thomas's knives that had microwelded carbide edges. He is the one credited with bringing microwelded carbide edges to the knife industry(the process has been used in the industrial sector for decades). I thought the idea was pretty darn good, but I hesitated to start making my own right away. At the time he was the only knifemaker making them, and I did not want to “steal” his idea. Fast forward to today where there are already several makers doing carbide edges, the process has been covered in the forums/media, and Warren Thomas has established himself as the pioneer of this field and has helped other makers get set up in I feel comfortable with using his idea.

I had read accounts of people making statements like “I haven't sharpened my knife in months, I use it daily on cardboard and it still cuts great!” or “After slicing with it for a few days it actually got sharper”. Now even though the knife industry is more down to earth than many industries, it is still a business, and people will say stuff to sell their when I hear such things I don't usually take it to heart until I've had personal experience. Over the years it seems like there is some new material/process that is marketed as the thing to revolutionize the knife industry. Most of them get their 15mins of fame, before moving on to the newest craze.
(our composite clad 15n20 shop knife, along with 2 cheap Chinese import knives, one of which is carbidized)
So I carbidized some knives and tested it myself. The first test I did was with a 2” blade from 15n20 heat treated to about 60rc. This was a shop knife I had been using for a good year so I was pretty familiar with its performance. I put a light carbide edge on it and started testing. The first thing I noticed is the knife did not get as sharp as without the carbide. I then got out cardboard and started slicing. For those of you not aware of it, cardboard is pretty abrasive stuff and will dull even good knives pretty quick. After slicing up a 5' long box into small pieces, doing hundreds of cuts, the knife did indeed get “sharper”. This is due to the abrasive cardboard wearing away the softer 15n20 steel and leaving more pronounced microserrations of tungsten carbide.
(our 2" carbidized shop knife after cutting up about 30sq ft of was actually cutting better than when we started)
I then proceeded to slice up half a dozen cardboard boxes, some of which were double walled. Through the entire process the knife cut cleanly except on pieces that had been water damaged and flexed too much, and lost their stiffness. I then went on to cut some 3/8” thick abaca rope. Abaca is a natural plant fiber found in SE Asia and is pretty abrasive. The rope I bought was also hung up in the open air market so I'm sure it had a lot of grit in it from airborn dust. I rubbed the rope against some steel, and sure enough, it scratched the steel (from the sand/dirt/dust in the fiber). The knife would not push cut the rope without applying excessive force, but it still cleanly sliced it. I cut a few hundred pieces before finally stopping because it was getting dark, and I was starting to get a blister.

(the knife is still zipping through the cardboard, 2 boxes done, several smaller ones left)
Simply put, the knife sliced much better/longer with the carbidizing. Since then we have been using it as our shop knife for a few weeks, and it is still cutting good with no sharpening to date. Not a chance you can shave with it, but it zips through cardboard boxes and various other items, no problem. Now keep in mind this is a very small blade, about 2" long. A longer blade would last longer due to having more edge to take the workload. For example a 3" blade would be 50% longer so you'd expect it to last 50% longer before dulling. A 4" blade would be twice as long so should be able to cut twice as much material, etc.
(all the boxes are cut, the knife still does fine on firm cardboard, even double walled, but will crumple the soft/water damaged portions somewhat when cutting them)

(snapped a pic while resting my fingers during rope cutting..its still slicing smoothly)
Next I bought some cheap Chinese kitchen knives to test carbidizing on. 2 were made from carbon steel and weren't too bad considering they were 50cents each. The 3rd knife was Chinese stainless steel 10” chefs knife for 75cents and it sucked bad.

(these knives were my first few attempts at carbidizing, and the coating could have been denser to increase performance)
I took the 2 Chinese carbon knives and carbidized one, but not the other. I then set out to see how well they would compare. The un-carbidized one slice-cut 52 cuts of 3/8” abaca before the edge slide. I tried the carbidized one and got close to 400 cuts before I got bored sitting there murdering innocent rope. It was still slicing fine.
I then carbidized the Chinese stainless knife and tested that. After carbidizing the edge I sharpened it and noticed it had much more of a burr. The steel was far too soft and was smearing. Thanks to the help of the carbides I got the knife sharp, and I did some free hanging rope cutting test with 3/8” abaca. It sliced cleanly through the half dozen times I did it....but upon inspection I noticed that even a light task like that was too much for the crappy Chinese steel.

(notice the shiny section on the edge. this is from the edge rolling over slightly in free hanging rope cutting test. this is due to the Chinese knifes steel being far too soft)
The edge of the knife was bending from the impact with the rope. Now this showed me 3 things, #1 is this 75cent stainless Chinese chef knife is complete crap, #2 is that even though the carbides are 70-80rc, due to the thinness of the coating they will bend with the edge before chipping off.(obviously as long as the blade steel would bend before chipping), and #3 while the carbides can make a soft steel cut better, it is no substitute good steel....a decent steel with carbidizing will outcut a crappy steel with carbidizing. I then decided to smack the stainless Chinese chef knife edge against our much thinner 15n20 Ecos knife. Even though both edges had carbide, the Ecos knife made a major cut/dent in the Chinese knife, with only a small mark on our edge that could be taken out with a honing stone. Its not very scientific, but its fun.
After trying carbidizing I will be using it on more of my personal knives. This stuff is great for slicing.