I recommend the sticky threads of New Members, especially
- the Newbie videos
- the thread with Library in its title (follow to a multipart guide)
3-piece ecig = battery + atomizer + cartridge (aka cart which is just plastic and stuffing for juice)
2-piece ecig = battery + cartomizer (aka carto which is a screw-on plastic and stuffing for juice w/ built in atomizer)
carts - probably cheaper if you take care of your atomizers, must be topped off often with juice
cartos - You can probably get a week out of each, maybe more with cleaning (doesn't always work well to clean them and if you run them dry and singe the stuffing then you can't fix that). Holds more juice than cart, you can vape longer without messing with it, may be more consistent vapor and may be better flavor
The most popular 3-piece 510 also has a variety of blank cartos available for it and a small selection of filled
The most popular 2 piece KR808D-1 (aka Vapor4Life and Bloogplanet) has a larger number of prefilled cartos and a smaller selection on blanks I think. To be a 3 piece it needs to use another PV's atty and carts (DSE901 atty fits right on, 510 atty would need an adapter)
Regardless of which you get, you are going to be refilling with ejuice
And I recommend manual batteries - I think most people on here do. Less work, more consistent hit, more protected against juice leaks, won't turn on by accident from wind or vibration