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Tension is a reaction force applied by a stretched string, rope, or other object on the objects which stretch it. The direction of the force of tension is parallel to the string, away from the object exerting the stretching force. So if an object hangs from a rope due to gravity, the gravitational force on the object points downward, and there is an equal tension force in the rope pointing upward, making the net force on the object equal to zero.
Note that tension can act in any number of dimensions and is not limited to one-dimensional strings. For example, the surface tension on liquids acts in multiple directions. Also two-dimensional membranes, such as a balloon, undergo a force that can be referred to as tension.
Because tension is a type of force, it has the same units as any force: kg·m/s2, or N.
Tension exists also inside the string itself: if the string is considered to be composed of two parts, tension is the force which the two parts of the string apply on each other. The amount of tension in the string determines whether it will break, as well as its vibrational properties, which are used in musical instruments.
The magnitude of the force of tension typically increases with the amount of stretching. For small stretching, the force is often described by Hooke's law.
String-like objects in relativistic theories, such as the strings used in some models of interactions between quarks, or those used in the modern string theory, also possess tension. These strings are analyzed in terms of their world sheet, and the energy is then typically proportional to the length of the string. As a result, the tension in such strings is independent of the amount of stretching.
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