The description about all these files and the precedence taken has been described very well in Administration Guide. Please go through it, it will clear all your doubts.
They can all do the same thing, but they all are used a little differently.
Exports is like the nfs exports file - here you can define who can connect to a server. So even if you have an entry in the users or users.local, exports might deny you a connection because you are not connecting from the right host.
Users and users.local do the same thing, except users is maintained by Bladelogic, via the acl push job. users.local is an override, you typically put in a Bladelogic administrator account here so that if you mess up your acl push, you still have a way to get to the box. Users.local is not touched by the acl push job. there is more on how these files work in the Bladelogic administration pdf.