A force is a pull or push of an object that causes it to accelerate. When there are more than one force acting on a body then we call this a system of forces.
Suppose that we have the following system of two forces:
Forces "a" and "b" are acting in opposite directions. If the forces have the same magnitude then the total acceleration in the object will be zero. When a system of forces produces no acceleration on an object we call this balanced force.
If the magnitude of the forces is different then by Newton's second law we have;
Where "m" is mass and "a" is acceleration. A system where the sum of forces is the product of mass by acceleration is called an unbalanced force.