An e-cigarette consists of a battery or a mod + one clearomizer/tank. A clearomizer consists of two parts: A container that holds the liquid (E-juice) and a coil that heats the liquid to vapor form. A sub-ohm clearomizer use a coil with low ohms (under 1ohm = sub) and produces both good flavors but also a way bigger vapor than a traditional clearomizer.
The coil is the part of a clearomizer that is consumable and lasts is usually 2-4 weeks depending on how, how much you use it, and with what e-juice. When the taste and/or vapor decreases, it is usually time to replace the coil. When you press the button on your battery/mod the coil is heated up and vapor is produced.
Below are the parts of an e-cigarette, the newer models are usually a little larger, but the principle is the same: