Lag screws are bolts with sharp points and coarse threads designed to penetrate and grip wood similar to a wood screw. They are generally used to anchor metal or wood to wood in places where you can't use a nut for a throughbolt or where an especially long bolt would be needed to penetrate a joint fully. Although lag screws do have unique and desirable applications, throughbolts are generally considered to be a more positive means of connection since they are less dependent on workmanship for reliable installation.