Basically, you’ve got three choices or Operating Sytems (OSes), with several flavors of each! Alphabetically:
1) Linux: this is a free version of the UNIX operating system which is availble for almost any computer platform. It comes in various “distributions” (“distros” or “flavors”). It is generally reliable, has fewer viruses, and lower hardware requirements but does not have all of the support from manufacturers and developers that the commercial operating systems. Well known versions include Red Hat Linux, SUSE Linut, Debian, Linux, and Ubuntu. I use Ubuntu as the main OS on my Laptop, which I use mainly for connecting to the internet. It boots up about 7 times faster than the OTHER OS on the machine.
2) Windows: this is the 800 pound gorilla. XP is the most popular version, followed by Vista, which is the current version or release. A new version called Windows 7 is scheduled for release on Oct 2, 2009. It should be more popular than Vista. Windows 7 is not the seventh version of Windows. I can think of a few Window 1.0, Windows 3.1 (there must’ve been versions 2.0 and 3.0 somewhere), Windows 95, 98, and ME. Windows 2000 (the first one worth using, IMHO). XP, and Vista. Not to mention Windows NT/Windows Server.
3) Macintosh OS X: Pronounced “O S Ten”, OS X appears in many incarnations but only runs on computers manufactured by Apple. It is actually built on another version of UNIX. I have no experience with OS X, but it is generally well spoken of.