Magic could be described as the power, art or ability to influence events, attain objectives or acquire knowledge through supernatural means.
These means or techniques "typically include chants and spells, gestures or actions that often have a symbolic relation to the desired result (for example, acting out a successful hunt of the past to make a future hunt successful), and the use of substances believed to have a special relationship with the powers needed to accomplish the intended purpose." (Encarta)
(See Magical tools and Magical techniques for more information)
Often a distinction is made between 'white magic' and 'black magic', where white magic is only used for good purposes. Black magic, on the other hand, is the use of magical powers for negative purposes, or for personal gain, i.e. in service to self, often, at the expense of others.
The word 'sorcery' is mostly used as a synonym of magic. Sometimes it is used for the use of magic or for having magical powers or abilities.