Gravity is a natural phenomenon by which all things with mass or energy are brought toward one another. Today, gravity is explained by two theories: Newton’s law of universal gravitation and Einstein’s general relativity.
Flat-Earthers treat the existence of the two theories as a supposed conflict, and they use it as “evidence” of wrongdoing. In reality, these are two separate theories that explain the phenomenon of gravity. Einstein’s is more accurate yet more complex. In contrast, Newton’s is simpler but less accurate. Despite having been superseded, Newton’s gravity continues to be used as an excellent approximation of the effect of gravity in most applications.
“Gravity” is the name of the phenomenon. Newton’s law of universal gravitation and Einstein’s general relativity attempt to explain the phenomenon. Newton’s law of universal gravitation describes gravity as a force, while Einstein’s general relativity describes it as a result of space-time curvature.
Newton’s law of universal gravitation is more straightforward and gives sufficiently accurate results for most everyday usage. General relativity is far more accurate but also much more complicated. General relativity can explain phenomena that cannot be explained using Newton’s law and addresses other phenomena not related to gravity.
There are very few cases involving gravity in our everyday lives on Earth that Newton’s law of universal gravitation gives an inaccurate result. It is why Newton’s law of gravity is still used in most situations in our lives.