AN OBJECT WILL RETAIN ITS CURRENT STATE OF MOTION -- INCLUDING BOTH ITS CURRENT DIRECTION AND SPEED -- UNTIL ACTED UPON BY AN UNBALANCED EXTERNAL FORCE.

Objects in our everyday experience tend to slow to a stop due to the forces of friction and air resistance. Eliminating these forces reveals that objects will continue to move as they are moving right now. Objects resist changes in this motion, due to their inertia -- a feature of the object's mass.