The cracked skin that you may get at the back of the heels is often a painful problem if it is allowed to progress. This problem occurs when the skin around the backs of the heel is dryer and thicker than it should ordinarily be. As the callus increases to create that dry thicker skin, it simply cracks as it is not very supple or flexible. The crack that occurs in the thicker skin, then attempts to tear or rip the good skin below it. In the worst cases, this will become painful, could bleed and be an for an infection, so does need to be taken seriously. The reason for these cracked heels is not totally clear. Some people just 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 is also a risk factor for this. Footwear that are open at the back are also considered to play a role in this problem.
The easiest method to deal with the cracked heels is to get a skilled foot doctor to remove the thicker hard skin and then use an lotion to make softer the remaining skin. You could try and remove that skin yourself with something like a pumice stone or file, but that is a lot of work and needs to be carried out a lot. The emollient ointment used after this ought to be put on consistently to keep the skin well hydrated and flexible. There quite a bit of opinion of what's the most effective lotion or emollient to use is and the optimum solution is the one that suits your skin. A bit of trial and error may be needed to get the best one. For cracked heels most foot doctors would suggest starting with a urea based lotion.