In geometry, a line starts by connecting two points, and then it continues infinitely onward in both directions. This is embarrassing, as geometric figures usually try not to get so carried away. Lines have only one dimension, their length. This feature makes them useful, but not necessarily relationship material. Lines exist in limitless areas called planes, which have no thickness, but are pleasantly filling if you're on a diet.
Rays are similar to lines in that they start at one point, and extend infinitely in one direction. Though a ray only extends one way, it is arguably as long as a line, since both are infinite. Do not bring this up with lines. They’re still mad about it.
Lines that connect two points, but do not extend infinitely in either direction, are called line segments, and are often praised for their restraint. Line segments can be connected to form intersections, polygons, or dirty words to impress your classmates.