What exactly is GUID? Why and where I should use it?

Question

What exactly is GUID? Why and where I should use it?
I\'ve seen references to GUID in a lot of places, and in wikipedia, but it is not very clear telling you where to

Answer 1

If you need to generate an identifier that needs to be unique during the whole lifetime of your application, you use a GUID.

Imagine you have a server with sessions, if you give each session a GUID, you are certain that it will be unique for every session ever created by your server. This is useful for tracing bugs.