Tennis elbow is caused by a strain to tendons in the forearm. The tendons become inflamed where they join the bony part on the outside of your elbow joint. Any activity that involves gripping and twisting of the forearm can cause this type of strain – most cases aren't actually related to tennis or any kind of exercise.
Golfer's elbow is a similar condition that affects the inside of the elbow joint near the funny bone.
Symptoms vary – you may have mild discomfort when you move your arm, or the pain may be bad enough to disturb your sleep. The outside of your elbow will feel tender to the touch and there may be swelling. You may also have pain further down your forearm.
Repetitive movements of the wrist will make the pain worse, especially if combined with a weight, for example if you're lifting boxes. However symptoms of your elbow can also be caused by problems in your neck.