In_The_Ear:
They range from very small to large.
Known as ITE aids.
Behind_The_Ear:
They sit on top of the ear and pipe sound down a tube.
Know as BTE aids.
Receiver_In_the_Canal:
They look like small BTE, but have a receiver (speaker) on the end of a wire that goes in the ear.
Known as RIC aids.
The BTE and RIC aids can have little domes on the end - or ear custom ear molds.
The aid that will work best for you will be determined by your hearing loss, dexterity, and lifestyle.