If we join two points, we get line segment.
We write line segment AB as (AB) or (BA)
Here, points A and B are the end points of the line segment.
A line extends in definitely in both directions.
A line as no end point.
We write line AB as (AB)
Ray is a line with one end-point.
Here AB is a ray.
It can extend from point B, but point A is fixed.
We write it as (AB)
(PQ) is a ray