This is the most common cause of shoulder pain. Bursa are the small fluid-filled pads that that act as cushions or shock absorbers between your bones and the tendons and muscles near your joints.
Bursitis by definition is the inflammation (itis) of the Bursa and is a very painful experience to suffer through. A good comparison of a bursa is to that of a blister. When the bursa has added pressures inside it become painful and unpleasant to use. It feels achy with moments of sharp stabbing pain when the joint is moved just right.
Tendonitis is the inflammation of the tendons that span the joints and hold the muscles in place. When the tendons become inflamed you will have spans of time where there is not any pain at all until you load the muscle or stretch the joint to its end.
Each ailment whether Bursitis or Tendonitis is unpleasant in its own respect. Treatments are geared to decrease inflammation, relax the pressure and return you to your normal level of functioning.