The facts that you need to know about lines are closely related to the facts that you need to know about angles. There are not many questions on the GRE that will specifically require you to determine properties about a line, but you'll need to know about various line relationships.

A line is always assumed to be a straight line, and it extends indefinitely in both directions. Lines can be designated by single numbers or letters, or they can be designated by two points that lie on the line. A segment is part of a line and is denoted by the two endpoints. Here, we show two lines and one segment, where p is the name of the first line, the second line is denoted by points AB, and the segment CD has a midpoint of F, which means that the distance from F to C is the same as the distance from F to D.

Parallel lines are lines that lie in the same plane and which never intersect. In the figure below, line p₂ is parallel to line p₃, but line p₃ is NOT parallel to line p₄ because if you were to extend lines p₃ and p₄, they would eventually intersect, as shown by the dotted lines.