Okay, here's the skinny: Ego's are a specific brand of 510 devices put out by Joytech, though there's all sorts of knock-offs. The different letters refer to different characteristics. The Ego-C has a 2-piece atomizer, which makes replacing them cheaper. The Ego-T is a
tank system: this means that rather than using a cartridge and atomizer, or a carto, it comes with a
tank. Type A and B are types of Ego-T; B has a larger tank than A. They all have a 650mahm battery and reliably fire at around 3.2-3.4 volts. There's also the Ego's that have larger batteries: the Ego-W and Ego-twist. These have, I think, 1060mahm batteries. The W has a special tank-like cartridge and I think it fires at 3.7, though i could be wrong. The twist is what's known as a variable-voltage device(vv). That means that you determine what voltage it fires at. Confused yet, lol?
Some explanations: mahm refers to battery size. The larger the mahm, the longer between recharges. Volts are the amount of electricity that the device puts out. The reason you care about voltages is that it determines what size atomizers and/or cartomizers you need to use. These are measured in ohms. There are discussion threads in this forum talking about these things.
Finally, all Ego's are what we call 510 devices. This simply means that they use 510 threads. This is important because it tells you what attys (atomizers? or cartos (cartomizers) to get.