Gravity is an attractive force. This doesn't mean it's pretty. What "attractive" means is that an object's gravity pulls other objects toward it.
The Earth's gravity naturally pulls us, and everything else, toward the center of the planet, which keeps us from drifting off into space.

The force of gravity is the force with which the Earth,Moon,or other massive body attracts an object towards itself.
The Earth isn't the only thing that has gravity. In fact, every single object in the universe has gravity.

Gravitational field is a region in which a mass experiences a force due to gravitational attraction.

here is a video showing what is gravity

http://www.youtube.com/watch?v=Uy3nATe85Kg