Physics teaches that a contact force is a force acting between two objects, or an object and a surface that are in contact with one another. Gravity and magnetic attraction are examples of non-contact forces, as they work over a distance. Non-contact forces still apply, even when there is a vacuum between the two objects.
Reference.com uses the example of two masses lying next to each other for contact force. If an individual places a force on one of the items, the contact force is proportional to the mass of the second item.
About.com attributes the concept of force to Sir Isaac Newton's three laws of motion. Newton described gravity as an attractive force between bodies that possessed mass. In Einstein's general relativity, however, gravity does not require force.
In physics, force is a vector quantity. In the SI system the newton is the unit of force, with 1 newton equivalent to 1 kilogram meter per second squared.
In a strict sense of the definition, contact forces are a simplification used in applications of classical mechanics. Modern physics teaches that everyday objects on Earth do not touch, but interact at the electron level at or near the object's surface, according to Reference.com