What do "thud", "live" and the other contact levels mean?

USA Football's Levels of Contact describe how much contact a drill involves:

• Air: no opponent, no contact.
• Bags: hitting a bag or pad, not another player.
• Control: set speed, the coach picks the winner in advance, contact above the waist, everyone stays on their feet.
• Control to ground: set non-competitive speed, predetermined winner, a controlled take-down is allowed.
• Thud: competitive speed through contact, no predetermined winner, above the waist, players stay up, quick whistle.
• Live: competitive speed in game-like conditions.

Only Thud and Live count as full contact, and those are the minutes USA Football recommends limiting each week.