I'd think that the words themselves pretty much describe their functions.
is a computer set up to "serve" information to other computers over a network. There are web servers, database servers, file servers, whatever. Unless you plan to host data for others to access, you won't need any of that. In the *nix world, most servers operate in text-mode only and may not even have graphic systems installed.
is just that, a computer you work on. It has the office, email, internet, and other programs that most people use every day. It can be stand-alone or connect as a client to one or more servers. The word used to refer primarily to high-powered office and engineering desktops, but that line is blurring as home systems become more powerful. So in your case, I think they're simply using workstation as a synonym for desktop computer.