Any phenomenon caused by the nontrivial vacuum state of quantum fields in the presence of boundaries, nontrivial topology, varying background potentials, spacetime curvature, ect, could be referred to as a Casimir effect. Besides the quantum fluctuation, at a finite temperature, Casimir effects can be significantly modified by the thermal fluctuation. This dissertation is devoted to the study of Casimir effects and their thermodynamical properties. Based on our research, we mainly focus on three topics, namely Casimir stresses and forces in inhomogeneous media, Casimir entropies, and classical and quantum frictions.