Final answer:
The main difference lies in the presence of forces; with no forces at all, nothing interferes with the object, whereas a zero net force implies that any present forces are balanced, causing no net change in the object's motion.
Step-by-step explanation:
The difference between saying that no forces act on a body and saying that the net force acting on the body is zero is significant. If no forces are acting on a body, this implies complete absence of any forces affecting it, which is a rare situation. On the other hand, if the net force on a body is zero, this means that all the forces acting on the body are balancing each other out; the body could still have multiple forces acting on it, but their vector sum is zero. For example, a book resting on a table feels the force of gravity pulling it down and the normal force of the table pushing it up; these two forces are equal in magnitude and opposite in direction, resulting in a net force of zero.
In the context of Newton's first law, an object with zero net force will maintain a constant velocity, which can include being still (velocity = 0) or moving at a steady speed in a straight line. This is distinct from a scenario where no forces act on the object, which would mean it's either permanently motionless (if it was initially at rest) or moving with constant velocity without any resistance or propulsion (which is practically impossible in real-world conditions).