Line
In geometry a line:

A line has no ends!
Play With It
Change the position of points A and B. Note that line AB extends forever in both directions:
Line Segment
If it does have ends it is called a "Line Segment".
Ray
If it has just one end it is called a "Ray"
This is Cool
Now play with this one ... point "B" is made to always be an equal distance from point "A" and "C". The result is a line (all the possible position of point "B" make a line):
Point, Line, Plane and Solid
A Point has no dimensions, only position
A Line is onedimensional
A Plane is two dimensional (2D)
A Solid is threedimensional (3D)