An ideal machine is that in which there is no dissipation of energy in any manner. In an ideal machine, the work output is equal to the work input, i.e., efficiency of an ideal machine is 100%.
In ideal machines mechanical advantage is equal to velocity ratio.

An Actual machine is that in which there is a loss of energy in some manner. In an actual machine, the work output is less than the work input, i.e., efficiency of an actual machine is always less than 100%.

In Actual machines, mechanical advantage is less than velocity ratio.

Reversible machine If a machine is capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as reversible machine. The condition for a machine to be reversible is that its efficiency should be more than 50 %.
Non- reversible or self locking machine If a machine is not capable of doing some work in the reversed direction, after the effort is removed, then the machine is known as non-reversible or self locking machine. The condition for a machine to be non-reversible or self locking is that its efficiency should be less than 50 %.

A machine having an efficiency greater than 50 percent, is known as reversible machine.

A machine having an efficiency less than 50 percent, is known as irreversible machine.

A machine having an efficiency equal to 100 percent, is known as ideal machine.