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1st-class lever
A lever where the fulcrum is between the effort and the load. For example, a seesaw or a pair of scissors.
2nd-class lever
A lever where the load is between the effort and the fulcrum. For example, a wheelbarrow.
3rd-class lever
A lever where the effort is between the fulcrum and the load. For example, tweezers and most human limbs (like your forearm) are 3rd-class levers.
Lever
A rigid bar that pivots on a fulcrum to move a load with an applied effort.
Mechanical Advantage
The ratio of output force to input force in a mechanical system.