As the name suggests, pod systems are e-cigarettes that make use of a “pod” as opposed to a traditional tank or atomizer. They ultimately do the same job: they hold the e-liquid and vaporize it to deliver vapour to you through the mouthpiece. They often have a smaller capacity than most tanks, at between 1 and 2 ml, although some pods are bigger. The pods simply push into place in the top of the device and pull out when you’ve emptied them or want to switch flavours.
The rest of the device is similar to any other e-cigarette. The batteries are fairly small in comparison to mods, usually having capacities of around 300 mAh, but they can be bigger than this in some cases. The “capacity” of a battery basically tells you how long it will last between charges. This means they have less battery than most vape pens (which tend to be around 650 mAh) but more than many cigalikes (which tend to be around 200 mAh).