Shoulder Question

Can the physiology experts please help explain a shoulder "impingement"? My frustruated interpretation is that it's not torn, not arthritis, but something ain't quite lined up correctly. I think I jammed it on a sprint finish and a few weeks off have not lessened the pain (arm raised 90 degrees laterally = ouch). Friend therapist says don't start anything remedial until the pain stops. Slowly going postal and gaining weight being out of the water..... I have an appointment with an orthopedist who has a sports leaning but am interested in other's experiences, too. Thanks, DV