First, ignore RAM, as it's not very popular.
Recordable media, whether CD or DVD, comes as either R or RW. R is recordable: it can be recorded only once. RW is erasable. You can burn to a RW, then later erase the disc and start again.
R discs can also be multi-task too, which means that if you specify it as a multi-task disc when you first burn to it, so long as the disc isn't full, you can then add to it (but not remove).
CD's are easier in that there's only one set of discs: CD-R or CD-RW. DVD's are more complicated in that you have + and - sets. The DVD- set was created by the DVD consortium, and is more 'official', and so most standalone DVD players people have under their TVs are more compatible with DVD- than DVD+. However, DVD+ is actually a better set, for technical reasons. DVD�R is also more easily readable than DVD�RW, so for making movie discs go down the DVD�R route.
So, assuming that your recorder can record to both, if you're making discs for playing back in order players, such as the one connected to your TV, it's probably best to go with DVD-R for compatability reasons. If you're wanting to backup your files on your computer, then go with DVD+R. If you want a disc you can erase and start again with, then ideally go with DVD+RW, unless your drives like DVD-RW more (compatability issues basically).