Balanced and Unbalanced Forces
When a wooden block is pulled from the two opposite ends with equal forces, it doesn't move. Such forces don't change the state of rest or state of motion of an object, and are called balanced forces.
In a situation where two opposite forces of different magnitudes pull the wooden block, the block moves in the direction of force with greater magnitude. Thus, the two forces are not balanced and the unbalanced force acting on the block produce motion of the block.
Examples
1- When a box is pushed on a rough floor, if the applied force is less than the frictional force (acting in the opposite direction of applied force), the box doesn't move. This happens because the frictional force balances the applied pushing force. When the applied pushing force exceeds frictional force, the unbalanced force produces motion of the box.
2- A moving bicycle slows down when pedalling is stopped, due to the frictional force acting in opposite direction of motion. To keep the bicycle in motion, one has to keep pedalling.
In the above situation, it appears that continuous application of an unbalanced force is required for maintaining the motion of an object, but this is quite incorrect. Any object can be moving with constant velocity only if the forces (external and frictional) acting on the object are balanced and there is no net external force acting on it. The unbalanced force acting on the object, changes either speed or direction of motion of object.
Therefore, an unbalanced force is required for producing acceleration of an object, and it would keep changing speed (or direction of motion of object) as long as this unbalanced force is being applied. However, is this force is withdrawn, the object would keep moving with the acquired velocity.
References
Textbook for class IX, NCERT Publications.
http://ncertbooks.prashanthellina.com/class_9.Science.Science/index.html