Well firstly what is a tendon, as opposed to a muscle or a ligament?
We have our skeleton of bones. These are moved about by muscles which expand and contract. The bones are joined together by elastic type fibres known as ligaments. The muscles are joined to the bones by less stretchy bonds called tendons. These are really extensions of the muscles themselves. They not only bond but also allow muscles to work at a distance by allowing lengthy connections, for example from the forearm muscles to the hand and fingers.
Doctors tend to describe muscles as being pulled, ligaments as being sprained and tendons as being strained. So, pulling a tendon is just a pretty serious strain on it.
We have about 4000 tendons in the body so there are plenty to pull. Those which are most at risk are those in the wrist area (if you fall and put your hand out for example) and in the leg and ankle area. Any sudden action which pushes a joint the wrong way or places stress on the connections between bones and their muscles will possibly pull the tendon.
The pull will probably cause considerable pain at the time. The fibres of the tendon are being torn. Some local swelling will occur as the body tries to protect and mend the tendon. This in itself will make the tendon less able to do its job and movement difficult.
More serious injuries can also occur in which the tendon is more completely torn or even ruptured altogether. The last condition will mean all links between the muscle and bones is lost and produce a serious immobility in the sufferer.
It is hard but important to distinguish between injuries to tendons and those to ligaments and muscles so seeking Orthopedic advice is recommended.