The rear deltoid head is located to the back of the shoulder, and is one of three heads which composes the deltoid (shoulder) muscle. It is also referred to as the posterior deltoid head.
The important point to note about the rear deltoid is its role during shoulder movements. Unlike the other two deltoid heads, it is not active during shoulder flexion or abduction, and as such is not recruited during overhead and flat pressing exercises, or front or side raises.
Due to its role being primarily to extend the shoulder, the posterior head is best targeted with exercises such as rows and rear flys. An athlete can either include these exercises within his/her shoulder session, or do what many others find more effective; look to target the rear deltoid head during their back session which involves similar movements.