I assume by "how different" you mean "how different from JK". Pretty different.
Basically you really have to adjust your mindset on how things are done. JK and Q3 have two different styles of editing. JK's engine is made to reject ANYTHING non-convex (except for 3dos), that is, all sectors, surfaces, etc. cannot have vertices at...strange angles. That's why you cleave stuff in JED, keeps things convex. Also, in JK, to add something (say, a set of stairs or a wall in the middle of a room) you must subtract it (delete it) from the world.
In Q3, everything is done with brushes. Think of it kind of like building a level with Legos. You start out in a total void in Q3, and you must add stuff into it to make the level. In effect you're filling things up. Once you get past the whole concept of adding things into the level to take up space, it's pretty easy to deal with. Q3 also has a much much much much much longer compiling time (compiling being a lot like gobbing a level, but it takes a hell of a lot longer), I've heard anywhere from 30 mins. to 4 HOURS depending on just how massive your level is.
Q3's engine is also much easier to deal with in that the only time you really have to deal with anything being convex is when you merge brushes (which is pretty much like merging sectors). Aside from that, doing really strange stuff like, say, a star, all you really have to do is get the brush in the correct shape and move it into position. Then, it's done. The only real thing that is annoying is the fact that even the slightest "leak" (that is, opening into the void that surrounds your level) can effectively kill your level until you remedy it. Fortunately most of the Q3 editors out there show you exactly where this leak when you try to compile.
I.. hope that helps..