Alternate Interior Angles
When two lines are crossed by another line (called the Transversal):
The pairs of angles on opposite sides of the transversal but inside the two lines are called Alternate Interior Angles. In this example, these are Alternate Interior Angles:

To help you remember: the angle pairs are on Alternate sides of the Transversal, and they are on the Interior of the two crossed lines.
Parallel Lines
If the two lines being crossed are Parallel Lines, then the Alternate Interior Angles are equal.
(Click on "Alternate Interior Angles" to have them highlighted for you.)