A penalty in football that occurs when a player uses his leg or foot to stop another player's forward motion. This is illegal and if a player is caught doing it a penalty results.
Tripping is usually a sign of desperation if a player is unable to block another player with his body. Blocking is supposed to be with one's body and head on. Tripping is considered unsportsmanlike conduct and results in a ten yard penalty against the offending team. Many times players who are on the grown already will use their legs or feet to trip a player as he runs by them.