As a man of position, I think that he probably had access to all sorts of brilliant minds and theories and seeing as this was the in the days well before copyright laws existed, I think that there is a fair chance that he passed of great swathes of stuff as his own.
Be that as it may, he is most famous for his laws of motion, which are:
Newton's First Law:
When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
In other words, objects are lazy. They want to keep on doing what they're already doing until something forces them to stop. This is true of physics and politics. A corollary of this is:
Newton's First And Halfth Law:
A student in bed will remain in bed until acted upon by a large enough panic.
Newton's Second Law:
The vector sum of the forces on an object is equal to the mass of that object multiplied by the acceleration vector of the object.
In short, Force = Mass times Acceleration. (F=ma). This true for both physics and religion. A Catholic church performing the Eucharist and undergoing acceleration, will require a greater force to stop it.
Force is not an energy field created by all living things which surrounds us and penetrates us. It certainly does not bind the galaxy together. I'm afraid that these Jedi and Sith people in that famous documentary series Star Wars, have confused Force with Duck Tape. Duck Tape has a light side and a dark side and holds things together. Incidentally, it's not proper Duct Tape unless it complies with AS 4254; otherwise it shouldn't be used for ducts.
If you do happen to be constructing a very large mass such as a DS-1 Orbital Defence Station (not a Death Star as some propaganda suggests) and accelerating it, then you might want to think about shields for your service ducts to prevent vandalism.
Of course, there is a negative corollary of this too; which states:
Newton's Minus Second Law:
Regardless of your speed and acceleration, Gary Oak will always be at least one step ahead of you.
The third law which Newton takes credit for is:
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.
This is exemplified by the poet Robbie Burns who wrote "If a body catch a body coming through the rye". In such circumstances, it is best to apply the laws of trespass for entering a rye field, then arrest the miscreant and take them away in the back of a Holden Caulfield (3.0L V6 Crew Cab).
Prior to these laws being passed in 1687, the universe was a very very different place. Gravity was owned by the monarch and as such, people who lived on the other side of the world were often in danger of falling off; hence the reason why to this day not many people live in Antarctica and why Australia was declared terra nullius (nothing terrible).
Thomas Hobbes, who was instrumental in passing the legislation in 1651, which was in force before Newton's Laws, specifically the Rule Britannia which categorically said that "Britannia Rules The Waves" is quoted as saying that the universe was "solitary, poor, nasty, British, and short". That description could have also been applied to Issac Newton as well; possibly a thief.