Atomizers are what make the e-cigarette device work. They heat up the E-Liquid and help to produce the great flavours.
Some atomizers are big, called rebuildable atomizers and the others are smaller, atomizer heads; both have the same job though.
More advanced users might be looking at the rebuildable atomizers as that is where the greater taste lies.
But both types of atomizer are a key element in all electronic cigarettes.