Well, since nobody else has touched this one, I'll try to give you the overview. Unix is a commercial operating system, and Linux is an open source variant of the idea. Meaning that thousands of programmers contribute to it.
They are similar in a lot of aspects (maybe TOO similar if you see some of the lawsuits over the last several years about Linux code that was directly ripped off from Unix).
Unix is still better for enterprise companies doing mission-critical deployments. But Linux is making a lot of inroads, and is fairly comparable these days.
On one hand, you have good, commercial software that is generally more reliable, and on the other hand, you have the open source (free, or at least cheap) version that gets a lot more movement with new features being developed, but no strict quality control.
Both work very well to do things like act as mail servers, web servers, or ftp servers, etc.