The job of tendons is to create movement.
They're essentially just extensions of the muscle.
Muscles turn into tendons which connect to bone. When the muscle contracts it pulls on the tendon which then pulls on the bone that it's connected to.
So think of doing a bicep curl - the bicep muscle contracts, pulls on the tendon (aka sinew) which attaches to the radius (one of the 2 long bones of the forearm) and pulls it upwards, bending at the elbow.
Muscles contain a relatively high proportion of elastin (a fibre that stretches up to 1½ it's original length, much like an elastic band). Tendons on the other hand contain only 2% of elastin, so whilst they stretch a little bit, it's not by much.
The main component of tendons, just like ligaments, is collagen which is very strong. In fact, gram for gram collagen is stronger than steel. Tendon are 86% collagen, so you can think of them like steel cables.
So elastic-like muscles change their composition to become like steel cables, sinews. It's these that pull on the bone when a muscle contracts.
Tendon injuries such as Tennis elbow usually occur though overuse.
Each tendon is encased in a sheath, much like a wire in a household plug.
Just like muscles, tendons grow with use. If they grow too much (because of doing the same repeated movement) then the tendon can become so big that it starts to rub on the insde of its protective sheath. This tendinopathy is accompanied by inflammation caused by the rubbing of the tendon against the sheath, and by pain.
Like muscles, tendons can also become ripped (though given the amount of collagen in tendon, it need a lot of force to rip a tendon). Tendon strains, can cause loss of movement and can take a while to heal.
If you've got a tendon problem, then you should first ice it for 48 hours and seek your doctor's advice. Usually this should be follwed up with treatment from a decent sports massage therapist who can help you with your recovery.
As ever, if you think I can help, give me a call.