The Exchange 2003 SDK has a list of the technologies and information about each one (examples of when you would want to use it, whether or not it's managed, required permissions, etc). This is a nice high-level overview to help focus you decide which technology is appropriate for your needs:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/e2k3/e2k3/_techsel_bytechnology.asp