DVD burning newbie question

[Jim Soriano]

Now that I've got a Pioneer 106 successfully installed in my Sawtooth 
(Dual G4-500, OS 10.2.8), I'd appreciate some insight as to the 
differences between...

a) DVD-R and DVD+R media?  I believe these are both write-once and are 
supported by my drive, but what are the other differences?  When do I 
use which?

b) DVD-RW and DVD+RW media?  I believe these are both write-once and 
are supported by my drive, but what are the other differences?  When do 
I use which?

Regards,

Jim