FreeBSD is a free, Unix-like, secure, high performance operating system mostly used as a server platform. It is a direct descendant of BSD. Other members of BSD-derived family are OpenBSD, NetBSD and DragonflyBSD.
There are some commonalities between FreeBSD and Linux as well as important differences. Both, FreeBSD and Linux are mostly POSIX-compliant.
Linux is just the kernel while FreeBSD is a complete, unified OS (kernel, libraries, user programs, etc). The kernel and the core user space are managed by the same core team. This leads to better compatibility between these two. In this aspect, FreeBSD is a more centralised and has a feel of being more methodically developed. Linux has distributions, often incompatible between each other e.g. with different package managers, different ways to configure files, etc.
FreeBSD is known for the excellent documentation (FreeBSD Handbook). It covers both basic and advanced topics using concise writing.
FreeBSD has some compatibility modules which enable the OS to run binary programs for other operating systems like Linux, SCO Unix, System V Release 4, etc.
FreeBSD usually supports 1-2 old major releases. Current FreeBSD release is 10.1.
FreeBSD contains both pre-built packages as well as a build system (called ports) for compiling and installing from source to allow package customisation.