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 happens when the skin around the backs of the heel is dryer and thicker than it should ordinarily be. When the callus increases to create that dry thicker skin, it simply cracks as it is not too supple or adaptable. The crack that develops in the hard skin, then attempts to tear or rip the good skin beneath it. In the worst cases, this will become painful, can bleed and become a portal for an infection, so does need to be taken seriously. The reason for these cracked heels isn't totally clear. Some people simply tend to have a dryer skin and some people, due to the way that they walk tend to build up the callus around the edges of the heel. Being overweight can also be a risk factor for cracked heels. Shoes that are open at the back are also considered to play a part in this condition.
The best way to deal with the cracked heels is to get a competent foot doctor to remove 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 such as a pumice stone or file, but that is a lot of work and needs to be carried out often. The emollient lotion used after this should be applied on a regular basis to keep the skin well moisturized and supple. There quite a bit of thoughts and opinions of what is the ideal lotion or emollient to use is and the best answer is the one which matches your skin. Some experimentation may be needed to get the best one. For cracked heels most podiatrists usually advise starting with a urea based cream.