Basically, you get an overall goal (mission type), like assassination, demolition, escape, infiltration, etc. You pick the overall area (hills, desert, snow, etc.) and the time of day. The game will then randomize a game for you with random placement of buildings, guards, etc.
This is in addition to the single player campaign and the multiplayer component.
I just played through three of them. It's pretty darn impressive, as each of these random missions takes 45 minutes to an hour to complete. It plays just like one of the levels, albiet no puzzles and no key-hunting, and no super-Ospreys.

This got me thinking... Any other game that can generate a random map for you to play on? I can't think of that many offhand, and many of them are quite old.