Trigger finger is a painful condition that causes the fingers or 
thumb to catch or lock when bent. In the thumb its called trigger thumb.
    Trigger finger happens when tendons in the finger or thumb become
 inflamed. Tendons are tough bands of tissue that connect muscles and 
bones. Together, the tendons and muscles in the hands and arms bend and 
straighten the fingers and thumbs.
    A tendon usually glides easily through the tissue that covers it 
(called a sheath) because of a lubricating membrane surrounding the 
joint called the synovium. Sometimes a tendon may become inflamed and 
swollen. When this happens, bending the finger or thumb can pull the 
inflamed tendon through a narrowed tendon sheath, making it snap or pop.
    
      
 
    
    
What Causes Trigger Finger?
Trigger finger can be caused by a repeated movement or forceful use of the finger or thumb. Rheumatoid arthritis, 
gout, and diabetes also can cause trigger finger. So can grasping something, such as a power tool, with a firm grip for a long time.
Who Gets Trigger Finger?
Farmers, industrial workers, and musicians often get trigger 
finger since they repeat finger and thumb movements a lot. Even smokers 
can get trigger thumb from repeated use of a lighter, for example. 
Trigger finger is more common in women than men and tends to happen most
 often in people who are 40 to 60 years old.
http://www.webmd.com/osteoarthritis/guide/trigger-finger