Cosmology is the study of the structure and evolution of the universe at large scales. In this thesis, the general principles of cosmology and also structure formation are studied. In Chapter 1 we consider homogeneous and isotropic universe. We also obtain Friedmann equations and we talk about content of the universe and different cosmic eras. In Chapter 2 we consider the primordial universe with inhomogeneities due to curvature and density perturbations and we compute the evolution of these perturbations. In this chapter, at first, we study the evolution of perturbations in Newtonian case and then in relativistic case. We also talk about conditions for transition between the two cases. Then, in Chapter 3 we consider higher-order perturbations and we compute the non-linear evolution of the fluctuations. These developments show how the current structures of the universe are formed. Bias concept is also introduced in this chapter. In Chapter 4, we use two different methods to obtain the relativistic equations of the evolution of the density perturbations of the cosmic fluid and compare them together. We also show that the results of the two methods come to the same at Newtonian limit.