Different types/styles of hearing aids:
Open Fit / Thin Tube / Receiver in the Ear:
These are very small behind the ear aids that attach to a thin tube that transmits the sound to the ear.
These are the smallest and fit inside the canal. They are virtually invisible. All are custom made.
These are somewhat bigger than the CIC and fit inside the canal. All are custom made.
These fill the entire ear and can be used for a larger range of hearing loss than the smaller CIC or ITC. The larger size makes it easier to adjust and handle. All are custom made.
Behind the Ear (BTE):
These fit behind the ear and provide more power. They are easily handled and adjusted. Sound travels to the ear through tubing to a custom made earmold.