As an end-user, you probably don't want or need subversion - as Renegade mentioned, it's version control for source code. (It can be used for other stuff than plain source code, but it's only really
useful for text-based files that change in predictable ways).
The tortoise part is a friendly GUI so you don't have to much around with the command-line.
Aaaaand... there's better systems around than subversion - if you aren't going to deal with a legacy project, you do
want when one of the newer DVCS
systems; not only is the 'D' part immensely useful once you wrap your head around it, but subversion's network protocol is hopelessly horrible