1. What is a circle ?
A circle is a type of line. Imagine a straight line segment that is
bent around until its ends join. Then arrange that line which
leads to the other end until it is exactly circular. Which in
conclusion, the distance along the line are the same anywhere
from the centre point.

2. Properties of a circle
1) Center : A point inside the circle. All point on the circle are
equidistant from the center point.
2) Radius : The radius is the center to any point of the circle. It is
half of the diameter.
3) Diameter : The distance across the circle. The length of any
chord passing through the center. It is twice the radius.
4) Circumference : The circumference is the distance around the
circle.
5) Area : A circle is a line so it has no area. Usually an area of a
circle is the area of region enclosed is enclosed by the circle.
6) Chord : A line segment linking any two points on a circle.
7) Tangent : A line passing a circle and touching it at just one point.
8) Secant : A line that intersects a circle at two points.

3. Other examples of circle related shapes :
Sphere
Cylinder

4. Formulas to calculate circle
Circumference - The circumference of a circle is the
perimeter / the distance around the outer edge.
Circumference = 2 Pi r
where r = radius of the circle and Pi = 3.142
Area = PI r2

5. Sphere formulas
Sphere surface area = 4 π r²
Sphere volume = 4/3 π r³

6. Cylinder formulas
Surface Area = (2 π r²) + (2 π r height)
Volume = π r² height = ¼ π d² height