hi,
those you have mentioned are effectively Closeup filters. As for the diopter ratings, simply put, the higher the diopter, the shorter the focussing distance, meaning you can get closer to get the subject, w/o throwing it OOF. (Note, its very hard for focusing at high diopteric values)...
For a starter, if you are shooting insects, go straight to a +4 first.
It should suffice, +2 is sufficient if you are taking flora.
Please, please, do NOT get the +10 by Hoya.
Not that its lousy, it is very restrictive and limiting. Rather than getting that, would recommend a +4,+4,+2 combination. Much more flexible.
Brand wise, cheap ones are Hoya, Tokina (haven seen them readily). These are single element glasses. Double element glasses, like the Nikon 6T, 5T,4T or the 250D, 500D are thought to be better, but personally have no experience with them. They are much expensive, hitting $100 and above.
If you are new to it, get Hoya +4 ($~15) and a +2 (~$10). It should be enough.
Regards