[Guide] This is to be used for step by step 'guides' showing you how to do something from start to finish, such as installing Apache, or SQL Injecting a website from start to getting a shell. [Article] Used when explaining information on a certain subject, usually in great detail, such as 'Ethical Hacking as a Career' this would contain things like description of the jobs, career paths, what to learn, professional workmanship etc.
[Request] When you need something such as, request a guide on xprobe2 or requesting a link to download this film etc.
[Source] Used when posting a source code of a program.