Top End Coil
A top end coil clearomizer is one where the atomizer head unit (the bit that contains the wicks and coil) is located at the top of the clearomizer unit (i.e. neared the mouth tip) like the CE4, CE6, Mini Vivi Nova. Most original clearomizers were built with a top end coil atomizer and sometimes when your e liquid gets low you need to turn the clearomizer upside down to allow the remaining amount of e liquid to soak into the wicks near the atomizer head.

Bottom Coil Feeder
A lot of new clearomizers like the MT3, KangerTech T3S, Pro-Tanks and so on now come with bottom end coil feeders which means that the atomizer head unit (where the wicks are) is located at the bottom of the clearomizer near the base. This helps to reduce the need for long wicks that soak up your e liquid and also allow for the e liquid in the clearomizer to be constantly in touch with the coil head wicks (no need to turn the clearo upside down when your e liquid is reducing).