The Macintosh's all used Apple's own O(perating)S(ystem), which is, at this point, called 'MacOS'. This system continued to be updated and modified until recently, when the original line was terminated with 9.2, and a new system, OS X (pronounced oh-ess-ten, Apple quickly adds), was made based on UNIX. Strangely enough, shortly after OS X, Windows XP appeared with a similarly updated GUI (but based on Windows NT or ME or something). Hmmm...
I have gotten a few applications. The coolest is 'Blender 3D', a neat 3D modeling program with an opaque I/O and no tutorial, but I also got the OS-X version of Angband. Angband is a Rouge-like game based on uMoria (Unix Moria). This game has itself spawned many subgames. I also have a OS 9.2 Playstation emulator, with which I have (nearly) beaten Dino Crisis 2 (Hints: To get large combos, run across a whole zone, dodging when the raptors jump at you, and pull out your shotgun and mow them down. Also, if you get the twin submacineguns for the female character (Regina?), then you can combo 15 of those water things over at Third Energy Reactor as you run down the walkway for 9000 points!). It runs well on Dino Crisis 2, but I cannot vouch for other games. It doesn't run in OS-X, only in 9.2.
Anyway, the iBook G4 is a superb machine. I don't do much that involves heavy processor power, but it runs smoothly most of the time, and it's far more portable than a iMac. I've played with using Photoshop on it, no problem, and I've watched movies a couple times. One of the idle fantasies I had when I bought it was that I'd write my first Novel on it, but anyone who knows me can tell you how that's going. Surprisingly, I haven't started gaming on this one; just more online comics. Then again, I've trailed off on the computer gaming since I've been at school, so that hardly applies. In short - I like the box, but I'm probably not going to do that much with it.