Neutron stars are the unique places in the universe in which matter with density beyond nuclear density can exist. Thus we analyzed the evolution of stars from birth to death in order to understand how neutron stars can be formed. Then we reviewed all the topics related to neutron stars from prediction and discovery to physical features and theoretical calculations. We also gave a review on various structures that can appear in the core of neutron stars. The structures that are suggested to be formed in the matter with density beyond nuclear density are beta stable matter, hyperon matter, kaon and pion condensation and deconfined quark matter. After that we gave a complete description about the creation of kaon in the nucleonic matter, in-medium features of kaon like effective mass and energy and Bose-Einstein condensation of kaon. As a main result of this thesis, is the Mass-Radius diagram for all the equations of state that we had for kaon condensation and other structures which were calculated with an algorithm that is based on dimensionless parameters in the equations of state and TOV equations. For example the maximum masses for GS1 and GS2 equations of state which are related to kaon condensation were determined. These maximum masses, found and ( Mass of sun) respectively. Eventually we compared these results with other maximum masses that are found for other structures and equations of state. We have explained about the physical concepts and in-medium effects of any structures and the reason of differences between the results. It should be noted that the computer program that we wrote for solving TOV equations in Matlab can be used for any equations of state and any structures.