Shells, sublevels and orbital shapes

The shell usually refers to the principal energy level, n. But within each n there are sublevels that describe both the 3-dimensional shape of the orbital that an electron occupies and the energy of that electron. These sublevels are sometimes referred to as subshells. For example, a shell, or n, might be 3, but its sublevels, or subshells, could be 3s, 3p, or 3d, which refer specifically to both orbital shape and orbital energy.

Orbitals -- basics -- atomic orbital tutorial - probability, shapes, energy