Newton's 3 laws were all rules about how things move. His first law was the law of inertia. We've actually already answered a couple of questions about inertia, so check out these two answers: Inertia Newton's First Law of Motion
His second law says that 'F=ma.' Yeah, this is an equation, but it's actually really simple. What it means is that if you put a force on something, it will be equal to that thing's mass (a bit like its weight) times its acceleration. (Acceleration is how much something is changing its motion.) So if the thing you're pushing on (putting a force on) is really heavy, then you won't change its motion very much. But if you push just as hard on something that weighs very little, its motion will change a lot in response.
His third law says that those forces (pushes) never are just from one thing (you, for instance) to something else (a wall, for instance). They always go both ways. So the force from the wall on you is exactly as big as the force from you on the wall, but in the opposite direction. This means that if you are pushing on something, it has to be pushing back on you.
It's often hard to see the effects of the 3d law because one of the things involved (the wall, for example) has such a big mass 'm' that it hardly accelerates at all. When two things with similar mass (say two billiard balls) bump into each other, it's much easier to see that the motion of BOTH changes, and in opposite directions. That's because they exert opposite forces on each other.
You should also take a look at the answer to the question Laws of Motion and Gravity
-Tamara (and mike)
(republished on 07/12/06)