E-cig clearomizers and tanks, are the devices that screw into the top of the battery and holds the e-liquid. They also contain the coil (heating element) that vaporises the liquid and creates the vapour that is inhaled.
The coils come in different resistances (measured in ohms) which slightly alter the vaping experience. The lower the resistance, the hotter the coil becomes. This produces slightly more vapour and can give more of a throat hit, but can reduce the e-liquid flavour slightly. High resistance coils heat to a lower temperature, which produces slightly less vapour and throat hit, but the flavour of liquid is usually enhanced.
Some clearomizers and tanks can be rebuilt and might contain replaceable coils. Replacing the coil when it eventually dies is going to save money over buying a whole new unit every time, but are generally slightly more expensive to purchase.
The life of the coil depends completely on how often it is used. Heavy usage will reduce the overall life and will need to be changed sooner. An average user can generally expect to get 2-3 weeks before burning out.