In mechanics we encounter several kinds of forces.
(I) Non-Contact Force
The gravitational force is all pervasive. Every object on the earth experience the force of gravity due to Earth. It is a non-contact force.
(II) Contact Force
A contact force on an object arises due to contact with some other object solid, liquid or fluid.
Example: Book resting on a table, friction force etc.
Contact force also arise when solids are in contact with fluids.
Example: A solid immersed in a fluid, Viscous Force (In fluid) etc.
Other common forces are tension in the string and force due to spring.
When a body of mass m is fastened with the string, then the weight of the body acts downward, while a force acts just opposite to the downward Force for balancing the downwards force. This force is called tension.
Tension, T = mg
where, T = Tension of the spring