The cracked skin that you can get at the back of the heels may be a painful condition if it is allowed to develop. This condition happens when the skin around the backs of the heel is dryer and thicker than it should typically be. When the callus builds up to create that dry thicker skin, it simply splits as it is not very supple or adaptable. The split that develops in the hard skin, then attempts to tear or rip the good skin beneath it. In the worst cases, this becomes painful, could bleed and be an for an infection, so does need to be given serious attention. The reason for these cracked heels isn't completely clear. Some people 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 sides of the heel. Being overweight is another risk factor for this. Footwear that are open at the back are also considered to play a part in this problem.
The easiest method to deal with the cracked heels is to get an expert podiatrist to eliminate the thicker hard skin and then use an ointment to soften the remaining 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 often. The emollient lotion used after this needs to be put on a regular basis to keep the skin well moisturized and flexible. There is a lot of thoughts and opinions of what is the most effective cream or emollient to use is and the optimum solution is the one that matches your skin. A bit of trial and error may be required to get the best one. For cracked heels most foot doctors would advise beginning with a urea based cream.