Clearomisers are basically e liquid tanks with a wick and a heating element, the wick soaks the liquid and draws it up to the heating coil where it heats the e liquid and releases the steam vapour
Clearomisers are the tops of the e cigarette that produce the vapour and hold the liquid.
Basic such as the CE4 are disposable and last around 4 to 6 weeks before the coil is spent and you replace the unit. When the coil is spent it will leak a small amount of liquid into the battery connector and produce less vapour.
Replaceable coil clearomisers like the Kanger T2, Kanger Evod and basically alll the other types have replaceable coils meaning you do not have to replace the whole unit just the coils.
Bottom coiled units such as the Evod mean the coil is in the base and some say this is better because the wick is always saturated.
Duel coil clearomisers such as the i clear 16 and aspire BDC means they have 2 coils therefore producing even larger amounts of vapour due to 2 burners.