Free Will is a being's ability to have control over its environment, future, and "destiny". A human being (presumably) has free will, and can therefore decide what to do; a rock does not have free will, and is a slave to the blind forces of physics.
Omniscience and Free Will
God is said to be omniscient, and this poses a special problem for free will: if God knows the future, that means that the future is predictable and immutable. This, in turn, means that our actions are predetermined. We may have pondered long and hard over which action to take, but the very act of pondering is as predictable as the execution of a complex computer program.
Note that this reasoning also applies to God: if God is omniscient, then he knows what he will do, and must inevitably do what he already knows he will do.
Christianity and Free Will
Many Christian sects, such as Calvinism, hold that it has already been decided at or before your birth whether you will wind up in heaven and hell. Your actions do not change your final destination; through your actions, you can only demonstrate which fate has been chosen for you. Many Calvinists do not believe in free will.
However, compatibilism is a philosophical position that holds that predestination and free will are compatible.
- Daniel Dennett, Elbow Room: the Varieties of Free Will Worth Wanting, MIT Press, 1984 (Amazon.com)