What is open?
Many players are unclear about what an open face looks like or how much to open the face. A good rule of thumb is to lay the club flat so that the grooves point to the sky and the leading edge points just to the right of the ball.
An open face is where the grooves of the club face the sky and the leading edge faces to the right of the ball.
What is square?
The leading edge of the club faces the ball. Square is useful for hard or wet sand in the bunker. It also comes in handy for a plugged lie or a ball that is in the lip of a bunker.
A square face is where the grooves of the club face above the ball and the leading edge faces toward the ball.