The general idea behind a globally unique identifier, or GUID, is that it is in fact, globally unique. This means that no two computers anywhere could possibly generate the same GUID. There is a program, called guidgen.exe, that (by using a weird algorithm based on time, MAC address, and a few other conditions; it is published) produces them. It's a 128 bit field, thus yielding a possible 2^128 number of guids. Guids are usually only generated by developers, so it's a fairly safe bet.

They are used in describing uniqueness, especially of controls in the registry. The GUID of the app server or control is built in, so for instance, Excel has the same GUID, no matter where you install it. (Excel.Application of Excel 2000, should be the same everywhere. Use regedit and some time and search for it). You can look in the ROT to see what objects are running. Microsoft has used them for other reasons that identify people or computers or what not, but generally they are used for CLSIDs of COM objects to avoid a namespace conflict (by using a huge numerical space).