No Problem, and actually a very good question!
There are many formats you are right: DVD-RAM, DVD-R, DVD-RW, DVD+R and DVD+RW
Those ending in RW, are of course the re-writable ones, corresponding to the same type as their 'W-less' counterpart.
DVD-RAM discs are fascniating, guarenteed for up to 100,000 re-writes in some cases. They are not meant for lasting data, but can be used for this in many cases. They are being used more and more in electronic devices such as camcorders. They are often encased as to protect the data more than the average DVD, which is a plus, they are also available in 4.7GB and double sided 9.4GB... oh the possibilites.
DVD-R and DVD+R are essentially the same... DVD+R is the newer of the two to be recorded to on a PC. These two have similar capabilities and results, and as far as compatability, most DVD recorders now support both. The standard for DVD players is DVD-R, but as the new models come out, you will notice that they also support DVD+R, as is the case in the last one i purchased.
DVD+R does have a leg-up on the DVD-R as time rolls on. MS Vista (code name Longhorn) has promised a drag and drop capability with DVD+R discs... and would they lie to us.... (ok don't answer that, but they didn't!)
Note that a disc that claims to be 4.7GB really only holds about 4.37GB, this is because the 4.7 GB claimed to be on the disc is actually taken as 4.7GB being 4.7 * 10^9 or 4,700,000,000 bytes, while your computer takes 4.7GB to be 4.7 * 2^30 or 5046586572.8 bytes (as a multiple of 2) thus:
5046586572.8 - 4,700,000,000 = 346586572.8 or approx 0.33 GB
*This being the same as the space you were missing.
