The cracked skin that you can get at the back of the heels can be 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 builds up to make that dry thicker skin, it just cracks since it is not very supple or adaptable. The split that happens in the thicker skin, then attempts to tear or rip the normal skin below it. In the most severe cases, this becomes painful, could bleed and become a portal for an infection, so does need to be given serious attention. The reason for these cracked heels isn't entirely clear. Some people just tend to have a dryer skin and some people, because of the way which they walk tend to build up the callus around the edges of the heel. Being overweight can be another risk factor for this. Shoes that are open at the back are also thought to play a role in this problem.
The easiest method to deal with the cracked heels is to find an expert foot doctor to eliminate the thicker hard skin and then use an cream to soften the rest of the skin. You could try and get rid of that skin yourself with something similar to a pumice stone or file, but that is a lot of effort and needs to be carried out a lot. The emollient lotion used after this should be applied on regularly to keep the skin well hydrated and flexible. There quite a bit of opinion of what is the perfect lotion or emollient to use is and the best answer is the one which matches your skin. Some experimentation may be required to find the best one. For cracked heels most podiatrists would advocate starting with a urea based ointment.