A basic screen targets the running back who starts out on the play lined up in the back field. It can be run to either side or the middle of the field. The running back either blocks or gives a look as if he will go past he line of scrimmage. He then turns to face the QB and wait for the pass behind the line of scrimmage.
The bubble screen is thrown to a player who is already lined up out wide to either the left or the right. The player turns and often takes a step or two toward the QB as the pass comes his way behind the line of scrimmage.
Both screens require the linemen to get out and block.