The main work of the hearing aid is to amplify the sound and carry sound from outer environment to ear making it audible to the wearer. Most hearing aids share similar components like microphone which grabs the sound, amplifier circuitry which increases the loudness, receiver or loudspeaker which delivers amplified sound into ear and batteries for operation.
The difference only lies in style, size and way they are placed in the ear. It ranges from small ones able to fit inside ear canal to medium sized that partially fits your ear canal to larger one that needs ear support to be placed. The smaller ones consume more battery, less powerful and more expensive.