The General Manager of a team is usually responsible for accquiring players, negotiating their contracts, assigning or reassigning the players, and trading them.
He also runs farm teams, supervises the arena and personnel, is responsible for hiring and firing the head coach and other support persons (such as trainers), and makes travel arrangements.
Before the 1970's, it was common for one person to be both the coach and general manager. That is not done today, since the responsibilities of both have grown enormously.