A Transversal is a line that crosses at least two other lines.

The red line is the transversal in each example:

Transveral Examples
Transversal crossing two lines
this Transversal crosses two parallel lines ... and this one cuts across three lines

Pairs of Angles

parallel lines angle example

When parallel lines get crossed by a transversal many angles are the same, as in this example:

See Parallel Lines and Pairs of Angles to learn more.