Exchange 2000 Server question

I am setting up my first Exchange 2000 Server – So far I have been pretty successful with it – What I am trying to do is not upgrade or Migrate from my Exchange 5.5 server but do a clean setup to weed out all the garbage and settings from 2 network upgrades ago – My question is this – Once I had exchange server installed and I wanted to setup clients – When you right click on the client name in AD you get the Exchange options choice – You can either create a mailbox or create an address – whichever one you chose you cannot chose the other unless you delete the one – What I want to know is what is the differnce between the two and when should you use which one?
Am I making sense here? If I have to do a migration I will but I would rathertry not to unless there is some reason I really should –

Thank you
Barb