There are many models and brands of electronic cigarettes. They have ecigars, PVs, rechargeable, disposable, ego, rebuildables, etc.
An ego, rechargeable, disposable all usually are composed of an atomizer(heating element/coil AKA atty), cartomizer(casing around atty which holds the eliquid), a battery (manual or automatic), and a casing around the whole device if its disposable.
A PV is a big model that delivers much vapor, with a similar design only bigger.
Ego has a higher battery capacity and output(amps) they deliver a lot of vapor for an on the go ecig. These almost always has a driptip which is like a nozzle u place in ur mouth to draw in vapor.
Ecigar is.. Well an ecigar lol, it tastes like a cigar only it's not.
A rebuildable is a model that has the most vapor output, highest battery compatibility, etc. only issue is, you have to clean it, replace batteries, coils, and perform maintenance regularly and they can range from $60-$200+