Bursitis is an inflammation or irritation (-itis) of a bursa, a small sac, sometimes containing fluid, which lies between bone and moving structures of the body such as muscles, ligaments and tendons. Bursae are kind of like little cushions which allow these structures to glide smoothly over bone.
There are apparently hundreds of bursae throughout our bodies, but most commonly bursitis affects bursae near a joint, particularly the knee, hip, heel, and elbow. The symptoms are pain and stiffness which is aggravated by movement; it is similar to tendonitis, which affects the tendons near joints. Sometimes bursitis (and tendonitis) are mistaken for arthritis, but unlike arthritis, bursitis (and tendonitis) are temporary, though they can become recurrent or chronic if their causes are not dealt with.
Bursitis is most commonly caused by injury or overuse during work or exercise. Miners get "miner's elbow" from repetitively swinging a pick; cleaners and carpet layers get "housemaid's knee" from kneeling for long periods on their knees. Those who exercise when they are not in good shape, have poor posture, or use their affected limb in an awkward position are also prone to bursitis. Very occasionally an infection within the bursa can cause inflammation, but this is rare. Bursitis also affects those with arthritis, gout, and diabetes. Bursitis can strike at any age.
To treat bursitis, first you've got to figure out what the cause is. If it's overuse or injury, you may have to avoid the offending activity. If it's an activity you have to do (say, for your job), you will need to learn how to position your limb better so as to avoid having the condition become chronic; wearing padding can help. Warm up well before exercise and stretch well after. Apply a splint, tension bandage, or ice to the affected area. Try physiotherapy to learn how to align your body better. Doctors will probably want to give you some kind of anti-inflammatory drugs (probably NSAIDs) or corticosteroid injections; these can relieve the pain but won't deal with the underlying causes of the problem.
Node your ailments!