Theory:

A circle:
When all the points on a plane is at a fixed distance from the fixed point is called a circle.
1(3) Ресурс 1.svg
 
The line that joins the centre of the circle and any point on the circle is called the radius.
 
Considering the definition of circles, the fixed point is the centre of the circle, and the fixed distance is the radius.

When all the points formed at the fixed distance from the centre of the circle gets joined, a circle is constructed.
 
2(4) Ресурс 1.png
 
A circle acts as a closure to all the points on the plane.
 
This closure can be classified into three categories:
 
1. The points that are interior to the circle.
 
3(4) Ресурс 1.png

2. The points that are on the circle.
 
4(4) Ресурс 1.png

3. The points that are outside the circle.
 
5(3) Ресурс 1.png
 
  
Some other components of the circle:
 
A chord:
When a line segment connects any two points on the circle, then it is called a chord.
6(3) Ресурс 1.png
 
A diameter:
The line segment that passes through the centre of the circle connecting two definite points on the circle is called a diameter.
The diameter is the longest chord on the circle.
 
7(2) Ресурс 1.png