Dru Lavigne says, FreeBSD is a UNIX® like operating system based on the Berkeley Software Distribution. While FreeBSD and Linux are commonly perceived as being very similar, there are differences:
Linux itself is a kernel. Distributions (e.g. Red Hat, Debian, Suse and others) provide the installer and the utilities available to the user. http://www.linux.org/dist lists well over 300 distinct distributions. While giving the user maximum flexibility, the existence of so many distributions also increases the difficulty of transferring one’s skills from one distribution to another. Distributions don’t just differ in ease-of install and available programs; they also differ in directory layout, available shells and window managers, and software installation and patching routines.
FreeBSD is a complete operating system (kernel and userland) with a well-respected heritage grounded in the roots of Unix development. Since both the kernel and the provided utilities are under the control of the same release engineering team, there is less likelihood of library incompatibilities. Security vulnerabilities can also be addressed quickly by the security team. When new utilities or kernel features are added, the user simply needs to read one file, the Release Notes, which is publicly available on the main page of the FreeBSD website.
FreeBSD has a large and well organized programming base which ensures changes are implemented quickly and in a controlled manner. There are several thousand programmers who contribute code on a regular basis but only about 300 of these have what is known as a commit bit and can actually commit changes to the kernel, utilities and official documentation. A release engineering team provides quality control and a security officer team is responsible for responding to security incidents. In addition, there is an elected core group of 8 senior committers who set the overall direction of the Project.
In contrast, changes to the Linux kernel ultimately have to wait until they pass through the maintainer of kernel source, Linus Torvalds. How changes to distributions occur can vary widely, depending upon the size of each particular distribution’s programming base and organizational method.
While both FreeBSD and Linux use an Open Source licensing model, the actual licenses used differ. The Linux kernel is under the GPL license while FreeBSD uses the BSD license. These, and other Open Source licenses, are described in more detail at the website of the Open Source Initiative.
The driving philosophy behind the GPL is to ensure that code remains Open Source; it does this by placing restrictions on the distribution of GPLd code. In contrast, the BSD license places no such restrictions, which gives you the flexibility of keeping the code Open Source or closing the code for a proprietary commercial product. Having stable and reliable code under the attractive BSD license means that many operating systems, such as Apple OS X are based on FreeBSD code. It also means that if you choose to use BSD licensed code in your own projects, you can do so without threat of future legal liability.