Relativity is a fundamental theory in physics developed by Albert Einstein that describes the relationship between space, time, and gravity. It revolutionized our understanding of how objects move and interact in the universe, showing that time can vary for different observers depending on their relative speeds and gravitational fields. The theory is divided into two parts: Special Relativity, which deals with objects moving at constant speeds, and General Relativity, which explains how gravity affects the fabric of space-time. Together, these concepts have profound implications for cosmology and our understanding of the universe.