Bursitis is inflammation or irritation of a bursa, a small sac located between a bone and muscle, skin, or tendon.
Tendinitis is inflammation or irritation of the tendon, which is a thick cord that attaches muscle to bone. DeQuervain's tendinitis results from overuse of the thumb tendons, often caused by repeated pinching with the thumb while moving the wrist. It can occur with activities such as writing, gardening, or fine handiwork.