A lot of this stuff, and in-depth explanations, can be found if you search the Blank Media forum.
Here's just a quick recap, as I understand it (so it's subject to correction by other members!) -
-R and +R are write once media - i.e. you can only write to them once, and you can't erase what you've written.
If you burn a multi-session disc, you can write stuff to it in seperate sessions, using an appropriate burning app (Nero, for example).
-RW and +RW - you can write to, then erase, then write to these discs more than once. How long they last is a matter of personal experience, and has been discussed quite a bit on here (in fact, there's a thread on this very page).
For the difference between + and -, have a look at the Sticky thread in this section.