It is caused by repetitive movements of the elbow joint. Contrary to the name, it does not only occur in tennis players, but rather in anyone who has repetitive stress to the elbow such as painters, butchers, carpenters and other sports enthusiasts.
The symptoms are primarily pain to the outside of the elbow. Over time, the area can get swollen and tender to touch.
Fortunately it heals over time if the repetitive motion is stopped. Rest, ice and anti-inflammatories can help cure it. Sometimes physical therapy and splints are needed. Talk to your doctor if you suspect you are suffering from tennis elbow.