Just to add to what's already been said a bit, the plain old Ego is essentially an Ego battery, with standard Joye 510 atomizers and carts. It's just like a 510, with longer battery life.
The Ego-T is the first of the 2 tank kits. Rather than cartridges with polyfill, it uses "tank" carts, with hold more juice. It uses a special atomizer which fits with the tank carts.
The Ego-C is very similar to the Ego-T. The carts are essentially the same, and the only difference is that rather than the atomizer and cone being a single piece (necessitating tossing the whole thing and replacing it when it goes bad, at about $15 each), the cone comes apart and a smaller, replaceable atomizer is stored within. These Ego-C atomizers can be had for around $25 for a 5-pack, so long term they are a bit cheaper than the Ego-T.
Any of the 3 use the same battery options (or nearly the same anyway), and all Ego batteries are compatible with any 510 atomizer/cartomizer/tank.
The Ego chargers are all the same as far as I have seen (4.2v 420mah output), and are all compatible with all Ego batteries (except for the passthrough cable of course, which only works with the passthrough battery). It's worth noting though that the Ego chargers are *not* compatible with standard 510 batteries, as they use a higher output current than standard 510 gear (even though they look exactly the same).