The cracked skin that you may get at the back of the heels may be a painful condition if it is allowed to develop. This problem happens when the skin around the backs of the heel is dryer and thicker than it should typically be. When the callus increases to create that dry thicker skin, it just cracks since it is not very supple or flexible. The split that happens in the hard skin, then tries to tear or rip the normal skin under it. In the worst cases, this will become painful, can bleed and become an for an infection, so does need to be taken seriously. The reason behind these cracked heels is not really entirely understood. Many people simply have a tendency to have a dryer skin and some people, due to the way that they walk have a tendency to build up the callus around the sides of the heel. Being overweight can also be a risk factor for this. Footwear which are open at the back are also thought to play a role in this disorder.
The best way to take care of the cracked heels is to find a competent foot doctor to eliminate the thicker callused skin and then use an ointment to make softer the remaining skin. You could try and remove that skin yourself with something similar to a pumice stone or file, but that is a lot of work and needs to be done often. The emollient lotion used after this should be applied on consistently to keep the skin well hydrated and supple. There quite a bit of opinion of what is the most effective cream or emollient to use is and the best answer is the one that suits your skin. A bit of trial and error may be required to get the best one. For cracked heels most foot doctors usually recommend starting with a urea based cream.