A force is a push or a pull, and it always acts between two objects. Some forces need the objects to touch. These are contact forces. Friction is a contact force that acts along the surface where two objects touch and opposes motion. The normal contact force is the push of a surface on an object resting on it, and it acts at right angles to the surface. The tension in a rope and the push of your hand are also contact forces.
Other forces act without any touching. These are non-contact forces. Each one acts through a field, which is the region where an object feels the force. The gravitational field around the Earth pulls objects towards its centre, for example a falling apple. The electrostatic field around a charged object pushes or pulls other charges, for example a charged balloon sticking to a wall. The magnetic field around a magnet pulls on magnetic materials such as iron, and magnets attract or repel each other.
Forces always come in pairs. When object A pushes or pulls on object B, B pushes or pulls back on A. The two forces of a pair are equal in size, opposite in direction and of the same type, and they act on different objects. A book on a table pushes down on the table, and the table pushes up on the book with an equal force. Each force in a pair can be shown as a vector, which has both size and direction.