Gravity is a force that attracts objects with mass towards each other. The weight of an object is the force on it due to gravity. There is a relationship between weight, mass and acceleration of free-fall (weight = mass × gravitational field strength). The gravitational field strength of the Earth is 10 N/kg.
A falling object accelerates because of the force of gravity, but as it speeds up, the frictional forces on it increase. Eventually, the weight of the object is balanced by the frictional forces and it falls at a constant speed. This is called its terminal velocity.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science