A mod is the battery unit itself. You then add attomizer or cartomizers, usually of your choice.
I started with a 510 kit, it wasn't enough to get me over the hump and got myself a couple of chucks which I like to use with 510 cartomizers because I want the simplicity and longer vape between fills. It's just a more powerful device. Some mods, like a chuck can be used with two different types of batteries (you supply the batteries that go into the mod-think of a flashlight) One type of battery is a lower voltage, 3.7 and is a good vape, the other is two batteries that will give you 6v. 6Volts vape hotter, gives you more throat hit and vaper but depending on the e-liquid you use, it tastes good or burnt. The nice thing is you can bounce back and forth depending on your tastes, the liquids, and your needs.
The batteries for the mods also last longer, at heavy vaping you will be recharging 510 units non-stop but with a battery mod unit you can pretty much vape all day if not longer without having to recharge.