Visit their web site to learn more. Of the BSD unix variants, I am most familiar with FreeBSD , currently running version 3.2 and 4.1. Other open source BSD descendants are NetBSD, OpenBSD, and the commercial BSDi. All of these are similar in nature, but each has a different focus. The name Linux is trademarked by the founder of Linux, Linus Torvalds.
More information is available at their site.
UNIX is a trademark of The Open Group. All other names and products, whether trademarked, GPL'd, copyrighted, or copylefted are the property of the respective owners. Any additions or corrections are welcomed.