to try and make it simpler there are in essence 4 types of electronic cig.
1. disposable cigolikes.. the kind of thing you see in petrol stations and the like.. look like a cig, e-lites being the common one. basic couple of hundred puffs then throw away one part and screw on another bit for another couple of hundred puffs. most are bloody horrible and can put you off vaping!
2. EGO's.. self contained battery units with a removable top section where the liquid goes. these generally are the ones under the £30 price range, can come in a variety of colours and some have a few extra features like USB pass through charging or adjustable voltage via a little dial ( vision spinner for instance). generally between 350 -1300 MAH power and they take screw on top part which can be an atomizer, cartomizer, clearmizer etc for your juice. some of them you can use for about a month then throw away ( ce3's ) others can have rebuildable coils.
main thing is the EGO style battery is a rechargable self contained basic unit.
3. VV/VW mods these are variable voltage/ variable wattage mods. can come in all shapes and sizes and have adjustable features ( changing wattage and voltage, have puff counter, ohm meter etc.) some are round tubes, some are box shaped mods. some have LCD or LED displays. but essentially the main difference between these and EGO style batteries is the batteries in these units are removable and generally a higher power.
4. last but not least Mech mods. in purest simplest terms, hollow metal tubes with no electrics. they just take a battery ( same as number 3.) but have no fancy electrics and can take a variety of juice tanks on the top. nomrmally for the more advanced vaper as you can with the different juice tanks make your own coils, wicking etc and get low/sub ohms for some big cloud vaping ( main thing to remember is battery section on ego's , VV,s and Mechs are different but all 3 can take the smae type of juice heads/tanks/clearos etc )
basic i know but probably covers 90% of ecigs