1. Eclipsed conformation : In eclipsed conformation, the C-H bonds on the front and back carbons are aligned with each other with dihedral angles of 0 degrees.In this form, the electron densities on the C-H bonds are closer together than they are in the staggered form. When two C-H bonds are brought into a dihedral angle of zero degrees, their electron clouds experience repulsion, which raises the energy of the molecule.Such type of repulsion is called TORSIONAL STRAIN.Further the atoms are closest to each other and faces Vander wall repulsion.The Eclipsed form has 3 kcal/mol. more energy than Staggard form. The eclipsed conformation of ethane has three such C-H eclipsing interactions, so we can say that each eclipsed C-H "costs" roughly 1 kcal/mol.
2.Staggered conformation : In staggered conformation, the C-H bonds on the rear carbon lie between those on the front carbon with dihedral angles of 60 degrees.In this form, the electron densities on the C-H bonds are farthest than they are in the eclipsed form. The repulsion between the their electron clouds are least . Further the atoms are also farthest to each other and Vander wall repulsion is also minimum.