Transversals
A Transversal is a line that crosses at least two other lines.
The red line is the transversal in each example:
Transversal crossing two lines

this Transversal crosses two parallel lines  ... and this one cuts across three lines 
Pairs of Angles
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.