Remember Last Page Visited... How?

[eluser]dhall[/eluser]
I am new to website dev so sorry if this is a basic one.
I am building a member site that has a page that can be viewed by the public with limited functions.
If a user goes to that page not logged in, then clicks on the LogIn, redirects them to the LogIn page, once they are logged in I want to take them back to the page they were just on.

How do I do that? I am sure it has some fancy name, but I don't know it.
Thanks for the help!