A line segment is a finite, unbroken portion of a line. A line extends infinitely in both directions; a line segment does not extend infinitely in either direction. Instead, a line segment ends at two points, called the end points of the line segment.  Alternatively, a line segment can be thought of as "the set of all points that lie on the line between" its endpoints.
For example, the real line extends through all the real numbers from negative infinity to positive infinity - it contains every real number. A line segment of the real line contains all real numbers between two certain points, such as all the real numbers between 4 and 5. Such a line segment is more commonly known as an interval, [a,b].