Custom hearing aids are molded specifically for the users ear. To order this type of hearing aid, an ear impression must be made and sent to the manufacturer. 

Behind-the-ear/Receiver-in-the-ear hearing aids fit behind the ear and have a wire or tube that sits flush to the users head. It attaches to a piece the fits into the users ear.

Each style/model has its own advantages/disadvantages and should be determined on a case-by-case basis. Factors that should be considered, when choosing the best style/model for the user, are severity/type/configuration of hearing loss, manual dexterity, lifestyle, cosmetic. It is best to speak with your A.uD/H.I.S. to determine which style is best suited for you.