A force is anything that changes an object's properties, such as speed, mass, momentum, etc.
The classical (Newtonian) definition of Force is: "A push or pull that causes acceleration." Since the introduction of Relativity, the possibilities have expanded. As a Force causes an object to accelerate toward the speed of light, its mass increases.
Net Force is the sum of all forces acting on an object. If there are 2 equal Forces and they are pushing in opposite directions, they would balance, so the Net Force would be 0, and the object would not accelerate. That does not mean that the object would not move. An object with 0 Net Force is said to be "in equilibrium." A skydiver that has been falling for a while is no longer accelerating. This can only be true if the downward pull of gravity is balanced by the upward force of air friction. The skydiver is "in equilibrium" but is still falling at a constant velocity.