As kanoodle mentions Haglund's deformity is a condition affecting the calcaneus or heel bone. Abnormal deposits cause it to protrude, irritating the surrounding tissues and frequently causing pain. There are several factors that may come into play when Haglund's deformity develops. Having a tight or short Achilles' tendon can compress the bursa found between your heel bone and your tendon. Having a high arch can force your calcaneus rearward causing incresased internal pressure and subsequent callus formation. While pumps are implicated be wary of: footwear with an overly stiff heel counter, poorly fitting shoes, work boots and athletic shoes with insufficient padding that may deprive your heel of protective cushioning and adequate room needed for your foot to move inside of your shoe.
Anyone suffering from the heel pain associated with Haglund's deformity can tell you that in addition to discomfort finding footwear is going to be problematic. Haglund's deformity causes a specific type of footwear related complication. The abnormal growth sticks out behind the heel, it must be avoided and/or accomodated if the individual is going to be comfortable and it can cause people who are trying to deal with this to buy shoes that are longer than their foot needs. Imagine taping a marble to the back of your heel and trying to fit that into a shoe. While a small amount of heel movement is desirable inside of a properly fitted shoe people with Haglund's deformity frequently find their heel wedged into shoes or they allow an excess amount of movement to occur which creates other problems for them.
Having a small bump on your heel may not seem like a big deal since it rarely interferes with foot function however this is a potentially serious problem that can interfere with your ability to walk without pain. Conservative treatments for Haglund's deformity include stretching the Achilles' tendon and I would include the plantar fascia which is the name for the tendon once it starts running beneath your foot. Good footwear is a must for anyone seeking to relieve the pain that can aggravate their every step. Even if the deformity is not painful it should be noted and accomodated since it is damage and may lead to future foot related injuries. Once you have the growth and conservative treatments such as rest, ice, stretching and anti-inflammatory medications have failed you may be a candidate for surgical correction.
This is not a particularly difficult surgery however anything that involves humans carries a degree of potential error and sometimes total correction is not possible. The two methods traditionally used include shaving down the heel bone or shortening the calcaneus by removing a wedge shaped piece. Sometimes surgeons will opt for keeping the calcaneal tendon attached even though that can be less effective than removal since there are more complications associated with tendon surgery. Athletes whose performance depends on having mobile joints and tendons may decide to play through the pain which is not a good idea. Similarly some people ignore the irritation before it becomes full blown pain which is also not smart. Bony growths are typically the result of your body trying to protect itself. If a particular area is under stress your body will rush additional blood and nutrients to the injured site and if the injury to the tissues persists the growth will keep pace with the demands your body is making.
Once acquired Haglund's deformity is an annoying, irritating, possibly expensive and painful condition that is not easy to get rid of. If you have rear heel pain determining the cause is going to help guide you towards a treatment plan. Healthy bones, joints, nerves, tendons and ligaments are in your best interest which is why stretching and lengthening measures should be practiced even by those whose heel bones are normal. Activity is good as it stimulates blood flow and circulation however over doing things is just as bad as under doing them. Find shoes that fit and be conscious of any foot pain, irritation or annoyance you notice as you move through the day. Some shoes with soft backs or clogs without backs may help prevent further discomfort. As kanoodle mentions there are also heel pads, lifts, certain shoe modifications and orthotics that may be used independently or collectively to alleviate your pain.
To summarize: certain people are more likely to develop Haglund's deformity than others. Educate yourself as to the foot and arch type you have. Listen to your body and remember that vanity and ignorance frequently contribute to minor problems that blossom into major ones. If your New Year's Resolution included stepping up your activity level remember that pain is different than soreness which should subside within a few days. Take care of your feet while they are flexible and strong because as many people can tell you, Haglund's deformity is a good way to discover that you didn't know what you had in terms of range of motion and pain free movement until it is gone.
Sources:
Basic overview
Patient complaints and discussions
Surgical reduction video