Theory of relativity is a combination of two different theories or says two different combinations. First, one is Einstein’s special relativity theory and second one is general relativity theory. Concepts included in theory of relativity consider space-time as a single unified entity. According to concepts of theory of relativity, following are the elements of unified entity of space-time:

Space and time (of course)
Relativity of simultaneity
Kinematic
Dilation gravitational time
Length contraction

Term of relative theory was firstly used by Max Planck in 1906. The whole theory of relativity is based on this term. Max Planck described how the principles of relativity theory could be applied on practical events and various theories. Later on, in the same discussion term Theory of relativity was used by Alfred Bucherer.

Relativity theory is considered a unified representative of multiple physical theories. Concepts from more than one physical theory are explained and broadly discussed in this theory, which makes this theory real broad and deep. During 20th century concepts and ideas of theory of relativity has transformed many theoretical concepts of physics and astronomy. In the most basic version of theory of relativity, 200-year old theory of mechanics was used as a base for further studies and descriptions. Isaac Newton is said to be the founder of theory of mechanics, not the whole theory but he developed and explained many concepts that are contained in theory of mechanics. The whole concept of gravity was introduced by him. In physics, theory of relativity contributed a lot to develop or improve the concept of elementary particles. Concepts of relativity theory were used to understand interaction of elementary particles more deeply. Theory of relativity also helped the world to move in the right direction to reach enhanced nuclear era. Not only in physics, have theory of relativity also contributed in fields of cosmology and astrophysics. Thanks to the application in theory of relativity, astronomical phenomena were predicted and eventually proved real too. These phenomena are consisting of following:

Neutron stars
Black holes
Gravitational waves

As described, Theory of relativity not only helps researches in physics, but also in cosmology and astrophysics. This broad theory became concern for cosmology since the basic concepts as black holes are explained by theory of relativity. Many concepts that are a part of astrophysics use many concepts and ideas of theory of relativity as a base for further researches and explanation. Being a solid combination of multiple theories, theory of relativity becomes a thing of concern for practitioners of physics, astrology, cosmology and astrophysics as well. In this book, main concepts of theory of relativity are covered. Both parts of this theory, which are general relativity and special relativity, are discussed broadly. This short introduction covers from history of this theory and to scope of the very same theory. It will provide a breakthrough in the broad theory of relativity that will help beginners to cover broad concepts easily.