According to physicists, there are 4 fundamentals forces that exists in the universe, and they are called the fundamental forces of nature.

Gravitation
Electromagnetism
The strong force
The weak force

Gravitation

You may be surprised that gravity is actually the weakest of the four fundamental forces, it is the force that bonds planets and stars and even galaxies together and it the the force that keep us away from falling of our planet, now how could it be the weakest ? It's simply that on a tiny scale gravity cannot overcome the three other fundamental forces, magnetism for example can make a piece of steel stick to it without falling to the floor as gravity would do. Gravity is the most familiar force to humans, we experience it every second in our every day life.

Newton Physics

Newton describes gravity as a force that attracts everything in the universe that has mass to something that have mass too, and these things could be anything from tiny particles to galaxies it's is a universal law that treats everything the same way, but the strength of it's attraction depends on the object's masses and the distance between them, the more the mass of an object the stronger the force of attraction gets and the longer the distance is the weaker the force will be.

Newton described gravity in his law of universal gravitation :

F : the force between the objects
G : the gravitational constant (6.674×10−11 N · (m/kg)2)
m1 : the mass of the first object
m2 : the mass of the second object
r : the distance between the center of the objects

Einstein Physics

Einstein describes gravity and different way, he consider other factors than just the mass of the object, Einstein describes gravity as a property of space and time, and he proved in the theory of general relativity he made that an object that have mass or energy can distort the fabric of space time and that's the origin of what Newton called gravity. In brief Einstein theory said that gravitation is the warp of space-time created by mass or energy. So that notion of energy made Einstein to discover that gravity can influence something that has energy but no mass like light if the force is strong enough.

An object that has mass bending space-time while causing gravity

Image Source

Force Carrier

For every fundamental force is attributed a particle, or a force carrier, and the particle that cause gravitation is called "Gravitons" it is a theoretical particle which is mass less, we have never been able to detect gravitons and they are predicted to be undetectable.

quick overview about the upcoming topics

Image sources : 1 - 2 - 3 - 4 - 5 - 6

References :

General Relativity

Newton's theory of gravity

Gravitons

Gravity