Filler Materials, Wicking Materials
Since the early forms of electronic cigarette when small prefilled capsules were filled with a sponge material which would hold the e liquid and feed the wick the types of filler or wicking materials has grown massively.
The most common forms of wicking materials are silica wicks which are used in most clearomizers and tanks and organic cotton which is used by vapers building their own atomizers.
Other forms of wicks include:
Stainless steel cable
Ekowool Braided Silica Cord
Aquarium Filter Foam
Some vapers now avoid using wicks altogether and drip directly onto the coil or use an atomizer with a well where the coils sit on the top of the liquid.