There are a number of things to consider when buy a hearing aid; chief among them are which type of hearing aid you need and what it will cost.
Here are the main types of hearing aids currently available on the market:
Behind-the-Ear-Hearing Aid - These are commonly called BTE, but can go under other names such as RIC (Receiver-in-the-Canal). As the name implies, this type of device fits behind the ear of the wearer and they use an earmold to connect to the ear. This design is great for people with mild to moderate hearing loss and is barely detectable unless looking directly at the back of the persons head. Can also accommodate high tech features many consumers want such as telecoils and the ability to connect wirelessly to Bluetooth devices.
In-the-Ear-Hearing Aid - Commonly known as ITE. This is probably what most people picture when they think of old school style hearing aids. These completely take up the ear canal and are very visible. They can be good for buyers, though, because they are easy to take in and out and leave a lot of room for tech features.
In-the-Canal-Hearing Aid - Also referred to as ITC. Very small yet easy for most people to get in and out, and also hardly noticeable once inside the ear. Downsides can be wax buildup and moisture issues and the smaller size can make it difficult for this design to have many features.
Completely-in-the-Canal - Called CIC in the industry, this is the tiniest model. It's all but invisible when placed into the ear canal, so it's great for people who are most concerned about others knowing that they wear hearing aids and are worried about vanity. Unfortunately, due to the fact that this design is extremely small, they can be hard to insert and remove for some people, especially those with arthritis. Cons are wax and moisture issues, much like the ITE not many features can be fit into the compact size, so some important features like directional microphones are not available. Battery life is also an issue, because the very small batteries don't last long.
A hearing test will let you know what your level of hearing loss is which will go a long way toward determining which hearing aid design is correct for you. For example, if you have a severe loss, you are going to want to go with a BTE model because it will provide the amplification you need. If you have a mild loss and you don't care about lots of bells and whistles such as those found on Bluetooth hearing aids you just primarily want something that will increase quiet sounds and decrease loud ones - a CIC model will probably be fine and also save you a good bit of money.