The system is what you choose to analyse motion.
`1.` According to system chosen, forces can be divided into two categories:

(i) Internal force (ii) External force

Whether a force is external or internal will depend on the system chosen. For example, earth is pulling a mass by applying a force mg on it. The mass is also applying a force mg on earth. If we chose earth-mass as system, then mg becomes an internal force as it is within the system. But if choose only mass mas system; then mg acting on the mass is an external force.
We do not show internal force in FBD while solving a problem.

`2.` According to the origin of force, we can classify it into two types :
(i) Field forces
(ii) Contact forces

The force experienced by an object without physical contact is known as field force.
e.g. Gravitational force, Electromagnetic force.

`(a)text(Gravitational force :)`

The force by which two bodies attract each other by virtue of their masses. If two particles having masses `m_1` & `m_2` are separated by a distance `r`, then the magnitude of force is given by `F = (Gm_1m_2)/r`. This is called Newton's law of gravitation.
The force acts along the line joining the particles, this force is always attractive in nature. For earth mass system, earth applies a force mg on mass `m`, which always acts towards the center of earth downwards. `mg` is called weight of the body.

`(b)text(Electromagnetic force :)`

If two particles having charges `q_1` and `q_2` are separated by a distance `r`, then the magnitude of force is given by: `F = (Kq_1q_2)/r^2`. This is called Coulomb's law and it acts along the line joining the charged particles. For like charges, the force is repulsive, and for unlike charges, the force is attractive.

The force experienced by an object by physical contact is known as contact force.
e.g. Frictional force (which will be dealt in detail later), Normal force, tension.

`(a)text(Normal force :)`

A contact force perpendicular (normal means perpendicular) to the contact surface that prevents two
objects from passing through one another is called the normal contact force.

`(b)text(Tension :)`

When a rope (string) is connected to a body and pulled taut, the rope is said to be under tension. It pulls the body with a force `T`, whose direction is away from the body and along the length of the rope. A rope is usually regarded to be massless and unstretchable. The rope exists only as a connections between two bodies. It pulls on the body at each end with the same magnitude of force `T`.

(i) If a string is massless, the tension in it is same everywhere. However, if a string has a mass, tension at different points may be different.
(ii) If there is friction between string and pulley, tension may be different on two sides of the pulley, but if there is no friction between pulley and string, tension will be same on both sides of the pulley.
(iii) If two persons are pulling a rope opposite to each other, each applying a force equal to `100 N`, then tension in the rope will be `100 N`.

`(c)text(Frictional force :)`

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.
The frictional force acts along the contact surfaces.