FreeBSD is a free Unix-like operating system much like Linux. The main difference is that, while the Linux kernel has been written from scratch, FreeBSD is based on the freely redistributable parts of 4.4BSD (Berkeley Software Distribution) known as 4.4BSD-lite. FreeBSD runs only on the Intel PC platform (i386 and higher); a port to the DEC Alpha platform is being discussed at the moment. Hardware requirements for FreeBSD are much like those for Linux.
The development of FreeBSD is more « closed » than the Linux development. A core team of developers makes the key decisions concerning the project. Big changes are discussed in advance on the mailing lists. The FreeBSD project has two development trees (just like Linux): « -current » and « -stable ». « -current » is where the development of new features is going on, while development of the « -stable » releases mainly are bug fixes.
FreeBSD can be used and (re-)distributed freely just as Linux. Most parts of the system are released under the BSD copyright; the rest is under the GNU GPL or the GNU LGPL.
You can find more information about FreeBSD (and download the whole system) at FreeBSD Inc.. The newest « -stable » releases and snapshots of the « -current » development code are sold on CDROMs by Walnut Creek CDROM (their web- and ftp-servers are running FreeBSD.)