If the resultant force acting on an object is zero, a stationary object remains stationary. A moving object keeps moving at the same speed and in the same direction.
If the resultant force acting on an object is not zero, a stationary object begins to accelerate in the direction of the force. A moving object speeds up, slows down or changes direction.
Acceleration depends on the force applied to an object and the mass of the object.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science