human
What are the most popular types of e-cigarettes and vape pens?
assistant
The most popular types of e-cigarettes and vape pens include tank systems, mods, pod systems, and open systems. Tank systems typically have a larger capacity battery that powers more powerful devices, while mods and pod systems usually have smaller batteries with less power. Open systems are designed to be used with regular e-liquids and do not require any additional hardware.