There are a number of BSDs (FreeBSD, NetBSD, OpenBSD, Mac OS X, etc.) Each is a separate project with its own flavor, based on the original BSD codebase. FreeBSD is performance-oriented, and x86-only; NetBSD is portability-oriented, and supports the most different processors; OpenBSD is security-oriented; Mac OS X is user-friendliness oriented.
Here is a decent web page comparing Linux and the various BSDs.