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.

Point

A small dot made by a sharp pencil on a plane sheet of papers or a tiny prick made by a fine pin on a paper provides us with examples that comes close to the idea of a point. A point has a position and its location can be ascertained.A point is named by a single capital letter of the alphabet and read as , say, 'point A', or 'point X'. The top of a cone, the corners of a cuboid and square are examples of points. points

Line

The straight crease in the paper  and the taut straight string are physical examples that come close to the idea of a line or, strictly speaking, the portion of a line.
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. lines

Plane

A solid has a surface which may be flat or curved. For example, the faces of a cuboid are flat and the surface of the sphere is curved.

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.
planes