Shells-->subshells-->orbitals-->spin number
Shells are the energy levels. The shells have subshells, which in turn have atomic orbitals. The orbitals are within the subshell. The azimuthal quantum numbers s, p, d and f designate the subshells. Consider the p subshell. It has 3 orbitals: px, py and pz. Each orbital can hold a total of 2 electrons, making 6 the total number of electrons a p subshell can house. The d subshell has 5 orbitals, and can hold a total number of ten electrons. The f subshell has 7 orbitals, and can house a total number of 14 electrons. Think of a shell as a neighborhood, a subshell as a house, and the orbitals as the different bedrooms within the house where the electrons stay. Hope this helps.