A line segment is part of a line, has fixed endpoints, and contains all of the points between the two endpoints. One of the most common building blocks of Geometry, line segments form the sides of polygons and appear in countless ways. Therefore, it is crucial to understand how to define and correctly label line segments.
If you had a line -- so if I drew in arrows which says that they're going infinitely, and you pick two points on that line and you said that, wait a minute, I'm way concerned about the points that are in between these two points right here, you would be creating a line segment. So line segment is only part of a line, and it has fixed end points. And by definition the line segment contains all the points that are in between your end points.
So to be more specific, if we had this line segment right here, A and B, A and B are your end points. So I'm going to draw the arrow right there and write that these are my end points. And the end points are important because that's how we label this line segment. So we're going to call this line segment A, B and notice I drew a little line on top of A and B with no arrows which tell you, the geometry student, that this is a line segment A, B that only goes from point A to point B and contains all the points in between.