Tackling is when a player on the defense stops the forward progress of the ball carrier by forcing them to the ground. When the ball carrier touches the ground the down is over.
A sack is when the quarterback is tackled for a loss of yards behind the scrimmage line.
Sacks are great for the defense because it increases the likelihood for a turnover on downs.
Down By Contact
Down by contact is when the ball carrier touches the ground and the scrimmage down immediately ends.
NFL Down By Contact
In the NFL down by contact happens when both knees of the ball carrier touch the ground.
NCAA Down By Contact
In the NCAA down by contact happens when one knee of the ball carrier touches the ground.