A force is a push or pull that acts on an object because of its interaction with another object. All forces are either contact forces or non-contact forces.
In a contact force the two objects are physically touching each other. Friction, air resistance, tension and the normal contact force are all contact forces. For example, the normal contact force is the push of a table up on a book resting on it.
In a non-contact force the objects are physically separated, yet they still affect each other. Gravitational force, electrostatic force and magnetic force are non-contact forces. A magnet can pull a paperclip towards it without touching it.
Forces always come from an interaction between a pair of objects, and each object feels a force. The two forces are the same size but act in opposite directions. Force is a vector quantity, so each force can be shown as an arrow, and the length of the arrow shows how big the force is.