Heyas Jaycea22,
510 and 808d are thread types (or connector types). They are the two most common connectors, as far as I know. You have to match the atty or carto type to the batt type (or buy an adapter). There's a greater variety of atties/cartos available for the510 thread (from what I've seen). After a couple pos local purchases, I ordered an ego, which has the 510 thread. Since then, I've tried probably a dozen different atties/cartos with it. I'm definately not disappointed with it.
I recently bought a mod with an 808d thread, and the 808d does have it's strong points also.
If you're looking for an ego type pv, you could do worse than to take a look at
E-Liquid and Electronic Cigarette Shop - Category Listings . Click the 20% off button in the upper left corner, and you'll find 510 big batt kits starting at ~$30 (for a riva 510). I've ordered from liberty flights a few times now and found their service to be great.
Hope this helps. Feel free to post any other questions
Maximillian the Vaguely Disreputable