There are pods that are pre-filled, it makes them easier to use, but you become reliant on other to make sure they keep producing them, and the flavour you like may dissappear and oftentime you have a minimal variety of flavours and nicotine strengths. These are called "Closed Pod Systems".
There are pods that you can fill yourself. These can be tricky to fill for older eyes, or shaky fingers - but give the widest variety of flavours and nicotine strengths.
There are pod kits shaped like a traditional or flattened pen? Some shaped like business cards a sherrif's badge and all other types of shapes and sizes.
There are pod systems that offer air-flow control and can be satisfying even to those who prefer direct lung draws and devices with a tight draw to mimic a cigarette.