What's the difference...

between Linux and Unix? Or is Unix just a general term that describes a style of OS? If not, then what makes pure UNIX as opposed to Linux. I know that Solaris is supposedly UNIX as well as FreeBSD (well it's supposed to be a UNIX "type"?). So is there a pure UNIX? Or is it all UNIX? Most importantly, when online game developers want people with UNIX skills what are they asking for specifically? I've heard that when you're running a large server UNIX is the only way go, but what system are these using? Bah ... most likely Solaris, but tell me if I'm wrong.

Just like there are standards for the C programming language, there are standards for UNIX. This is known as the POSIX standard, and defines much of how a UNIX system shall be designed. This standard is a good thing, as being proficient on one flavor of UNIX (say, HP/UX) allows you to adapt quickly to other flavors (Solaris, AIX, etc).

Linux is POSIX compliant, but it was derived from Minix instead of the original AT&T UNIX developed in 1969. For this reason, Linux is not considered to be a "true" UNIX operating system.

a big difference between most unices and linux is its open source properties. information about the open source movement can be gathered at gnu.org under the why link. but if you're not interested in reading that much:

linux programs are mostly free. no company can copyright them. these programs are called copyleft. (further explaination can be found at gnu.org). GPL is the license that copyleft programs are distributed with. Linux is a gpl'd operating system. it is free for all uses (except illegal ones). it can be downloaded for free or bought.
most other unices are like windows in that they are owned by a company, and sold for profit.