A block is the nautical term for what is commonly called a "pulley".
A block consists of one or more sheaves inside a frame.
The term "block" usually refers to a block that is attached at one end by a shackle or other fitting and can turn and rotate, alowing a line to enter the block at a variety of different angles.
A turning block is a block that is fixed to the deck, or some other part of the boat, and does not move. The line must enter the turning block in a plane that is parallel to the block's sheave.