To elaborate on what
SirDice said, the difference between the two is that of
tweaks and configurations.
PC-BSD has configurations for on-the-fly USB mounting and a lot of background scripts to make the overall functionality more user-friendly.
Example: in FreeBSD, you can either download the binary of a program via the
pkg_add command and install it. However, since such binaries are compiled to be as compatable as possible for a specific architecture (e.g. i686), it can function either slower or faster on your machine - depending on its parts and configuration. For most users, this is not a concern. However, the recommended approach to program installation in FreeBSD is via building the package from the "
/usr/ports" repository. That way, you are more or less guaranteed the
maximum performance of that custom compiled package on your machine. However, to the average user, this is not of paramount importance.
PC-BSD takes a different approach to this, and it is here where FreeBSD and PCBSD diverge. If you use Windows you are familiar with the double-click on an exe file, install and voila - you have an installed and working program. PC-BSD has a similar concept called a
PBI, where you can double-click and an installer pops up and you continue much as you would in Windows. Any software installed using PBIs are installed in a separate directory "
/Programs". Contrast this to FreeBSD where the software - downloaded / compiled and installed - are usually stored in "
/usr" (as in most Unix-like systems). And, since PC-BSD is also FreeBSD, you can also: 1) compile and install using ports; and 2) download and install using
pkg_add.
In summary, IMO technologically,
the difference between the two is one of a matter of design philosophy: PC-BSD focuses on the desktop and the average user by making it easy to use the system with a number of tweaks (outlined above), while FreeBSD focuses (arguably) on the server by ensuring stability, security and through a design and promotion of its ports system, maximum performance.
Note, because
PC-BSD is inherently FreeBSD at the core,
it has all the benefits of FreeBSD open to it. However, IMO the implementation of the PC-BSD system, somewhat forgoes that philosophy, but makes for an easy to use system, and one that a regular user can use without any qualms. FreeBSD, on the other hand, IMO does require some work. But, the beauty of the FreeBSD option is that
by proxy you learn a whole lot more about the system and Unix on the whole. And that, to me, is awesome.