Carpenter pants are the only pants I know of that went to trade school. You never hear about plumber pants or electrician pants or mason pants (Freemason pants notwithstanding), but carpenter pants are so popular that they can be found both on and off the construction site.

Carpenter pants have a distinct look to them, either canvas or denim with extra-large pockets, wide, straight legs, two smaller pockets halfway down the right leg, and the infamous hammer loop halfway down the left thigh. To detail the key features:

Canvas and denim are both well-known for their durability, originally intended for manual labor. Denim jeans are popular off the worksite as well because they're durable, comfortable, and easy to clean.
Large Pockets
Carpenter pants have very large, deep pockets for holding nails, tools, or tape measures, or keeping your hands warm on cold mornings. The ubiquitous fifth pocket is of course present, although nobody knows what it's for. I keep a Fisher Space Pen and a Mag-Lite Solitaire in it.
Wide Legs
Carpenter pants have wide, straight, baggy legs for maximum maneuverability. They don't bunch up uncomfortably or restrict your range of motion in any way, important when crouching, bending, or contorting on the jobsite. Not everything you have to work on is in a convenient place. In fact most things aren't.
Extra Pockets
Although they don't have as many pockets as cargo pants, there are two smaller, nested pockets halfway down the right leg. I'm not sure why they're on top of each other like that, but they're handy for long, thin tools like screwdrivers, drywall rasps, and chisels. They're slightly too thin and deep to reach into so I don't recommend keeping small items like nails in there.
Hammer Loop
Never in human history has there been a more poorly designed tool hanger, and yet it has achieved a level of popularity that belies its uselessness. The ridiculous part is that everyone knows it's useless for holding a hammer, or anything else for that matter. Actually it's all right as long as you don't walk more than ten feet, but the hammer loop is so unnecessarily large that any substantial swinging or sudden motion will cause it to fall right out. And yet they continue to be designed the same way, year after year, in every brand.

Other than that, carpenter pants are rugged, comfortable pants ideal for a wide variety of jobsites that require multiple, spacious pockets for tools and fasteners, when you don't have quite so many to require a tool belt. If they would just fix the dang hammer loop they'd be almost perfect. Almost perfect... I'm not sure it's possible to fix plumber's crack.

Update: I've received a few comments from my fellow noders that the hammer loop is not actually intended to be used to carry your hammer around. Where they found the documentation on the hammer loop I have no idea, and how this justifies the design choice has been left unclear. But the limitations of the hammer loop are well-known and it's only used as a place to momentarily hold a hammer when switching tools. However, to some people, it's also a convenient place to clip a carabiner for holding other tools.