Atomiser

Some companies use separate atomisers as opposed to cartomisers (which contain the atomiser).  An atomiser does the mechanical work of turning the nicotine into the vapour - it is the 'filter' but without the nicotine.  You have to add the nicotine by way of a separate cartridge, alternatively a 'tank' (see below).  Atomisers often have short life spans and can be expensive to replace.

Recommendation: Go for a simple two-piece configuration (Battery + Cartomiser) rather than a three-piece (Battery + atomiser + tank/cartridge)