Relativity is a branch of physics developed by Albert Einstein in the early 20th century that deals with the relationship between space and time. There are two main theories of relativity: special relativity, which describes the behavior of objects moving at constant speeds in relation to each other, and general relativity, which explains the force of gravity and the curvature of spacetime. These theories have revolutionized our understanding of the universe and have led to numerous groundbreaking discoveries in cosmology, astrophysics, and particle physics.