Alternate Interior Angles
When two lines are crossed by another line (called the Transversal):
Alternate Interior Angles are a pair of angles on the inner side of each of those two lines but on opposite sides of the transversal.
In this example, these are two pairs of Alternate Interior Angles:
And

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
When the two lines being crossed are Parallel Lines the Alternate Interior Angles are equal.
(Click on "Alternate Interior Angles" to have them highlighted for you.)