The cracked skin that you can 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 typically be. When the callus increases to create that dry thicker skin, it just cracks because it is not too supple or adaptable. The crack that happens in the hard skin, then attempts to tear or rip the good skin under it. In the worst cases, this will become painful, could bleed and become a portal for an infection, so does need to be given serious attention. The reason behind these cracked heels isn't entirely clear. Many people simply tend to have a dryer skin and some people, because of the way that they walk have a tendency to build up the callus around the sides of the heel. Being overweight can be another risk factor for this. Footwear that are open at the back are also considered to play a role in this condition.
The easiest method to deal with the cracked heels is to find a competent foot doctor to eliminate 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's a lot of work and needs to be done often. The emollient lotion used after this ought to be put on a regular basis to help keep the skin well moisturized and flexible. There quite a bit of opinion of what is the most effective lotion or emollient to use is and the best answer is the one which matches your skin. A bit of trial and error may be required to get the best one. For cracked heels most foot doctors usually suggest starting with a urea based cream.