Newtons 3 Laws of Motion are as follows...

1. The Law of Inertia - "A body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force."

This means, an object tends to 'keep doing what its doing', until 'disturbed'. The vector direction of the external force is applied, with respect to the motion/state of the obect, will determine whether the object stops, accelerates, changes direction, or deccelerates.

Lets examine a few examples:

A book is on a flat surface, this book will continue to 'doing what its doing', which is, remaining on that surface, unless the normal force of the surface can no longer maintain its balance with gravity. A more dynamic example...

Imagine a ball in a vacuum, outer-space - If a force is applied on the ball, it will maintain the energy of that force/its vector direction, until eternity. The ball will move for ETERNITY, because its in a vacuum, where no external forces can disturb it.

Lets return to that book.. What would happen if you applied a force to the book, sliding it across the surface? Why?

2. F=ma - (Force equals mass, multiplied by acceleration).

Simple as it sounds, many people are confused by this most basic principle, we should start with an example...

An object with a mass of 10g, and an acceleration of 5m/s^2 had a force of 50N acted upon it.

To solve the problem, we just multiply 10 by 5, its that easy. Another thing which should be known, an increase in "a" is inversely proportional to an increase in mass. What does 'inversely proportional' mean? Basically, when one increases, the other decreases.

Obviously, real life scenarios aren't this simple, sometimes you may have the quantities of "m" and "F", but not "a". Other times, "F" and "a", but not "m". This isn't a problem, as long as the value for two of the 3 parameters are known, you can solve for every other parameter, algebraically - F=ma, a=F/a, a=F/m

Heres a problem:

What is the mass of an object with an acceleration of 30m/s^2, when the Force(applied) was 100N? Use the formulas I provided above.

3. Action/Re-Action - For every action, there is an equal and opposite reaction.

Basically, If I push a wall with a force of 30N, it exerts that same exact force of 30N back onto me.

This can be illustrated in swimming... You push water in the opposite of your intended direction, the water exerts a force back onto you. Unfortunately, humans are very inefficienct swimmers, so the majority of energy applied against the water, ISN'T converted into 'work' - That has more to do with how our bodies are designed, than anything else.

Quite possibly the simplest of all 3 laws, yet a very fundamental one which can be observed in the behavior of waves -more specifically, 'reflected pulse inversion'-.

You should now have a basic understanding of Newtons Laws of Motion, but remember, 'for every action, there is an equal and opposite re-action'; I don't know why I said that, but it felt appropriate as a closing statement, :).