The cracked skin that you may get at the back of the heels can be a painful condition if it is allowed to progress. This condition happens when the skin around the backs of the heel is dryer and thicker than it should normally be. As the callus builds up to create that dry thicker skin, it just cracks since it is not too supple or adaptable. The split that occurs in the hard skin, then attempts to tear or rip the good skin beneath it. In the most severe cases, this becomes painful, might bleed and be a portal for an infection, so does need to be taken seriously. The explanation for these cracked heels is not really completely clear. Some individuals simply have a tendency to have a dryer skin and some people, due to the way which they walk have a tendency to build up the callus around the periphery of the heel. Being overweight can be another risk factor for this. Footwear which are open at the back are also thought to play a part in this condition.
The easiest method to take care of the cracked heels is to find a competent foot doctor to eliminate the thicker hard skin and then use an ointment to soften the remaining skin. You could try and remove that skin yourself with something such as a pumice stone or file, but that's a lot of effort and needs to be done a lot. The emollient cream used after this has to be applied on a regular basis to keep the skin well moisturized and flexible. There is a lot of opinion of what is the best cream or emollient to use is and the best solution is the one that suits your skin. A bit of experimentation may be needed to find the best one. For cracked heels most foot doctors would suggest beginning with a urea based emollient.