You can use any USB-based keyboard on a Mac1. Nearly all keyboards sold these days are USB, but if yours is PS/2 then it won't plug directly into the Mac, since they don't have PS/2 ports.
You can buy a simple adapter to use PS/2 peripherals on a Mac (like this inexpensive one from Newegg).
Here are some key mappings for how the special PC keyboard keys translate into special Apple keys:
You might also want to have a look at MacWindows.com's hints for sharing a keyboard and monitor.
1While any USB keyboard will function on Mac in terms of basic usage, a given keyboard (e.g. from Logitech or Microsoft) might require driver software from the keyboard's manufacturer to enable the use of special media keys, or other features that go beyond basic usage, so check before you buy. This caveat also applies to mice.