I am trying to figure out
1. What is the difference between DVD + and DVD - R . I always heard to buy - because more players could play them.

2. What kind of DVD RW should I get for testing my videos on DVD without having to waste DVDs?

If you really want to know the difference between + and - formats go here:
DVD+R - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/DVD%2BR)
DVD-R - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/DVD-R)

Consensus is that -R is still the more compatible of the two formats. That's all I use.

As for the right RW you should use, the answer is whatever works on your gear. RW (+ and -) are both less compatible than either of the R formats. You may have a little trial and error ahead.

Here's something you might want to consider. The R media I use burns at 16X. The RW media I just checked from Staples.com only burns at 4X with one only 2X. Is your time worth the $.20 cost for burning a coaster? I've never used RW and I'll throw away one DVD for about every other project I do. It's generally for the review copy where I find something I need to change in the project.