In order to understand geometry as a subject, its important to understand its basic concepts:points, lines, planes etc. This will enable to understand more about geometry , particularly of figures with which we are mostly familiar.
In geometry, by a line we mean the line in its totality and not a portion of it. In practice, only a portion of a line is drawn and arrowheads are marked at its two ends indicating that it extends indefinitely in both directions. A line has thus no end points. It can be named using a single point as well as by two points.
Flat surface are known as plane surface and the other surfaces a curved surfaces.
A plane is a surface which extends indefinitely in all direction.
In geometry, by plane we mean the whole plane and not just a portion of it. A plane is commonly named by taking three points on it which are not on the same line.