We've identified 2 websites which give you an introduction to the different parts of each instrument as well as tips on what to look for when choosing an instrument within your intended category. They are listed below by instrument.

Categories
Instruments are designed and manufactured to fit into 3 main categories:

Beginning
Beginning instruments are designed to be cheap (price-wise) and sturdy. While both of these qualities may seem good, they often mean that major compromises have been made in meeting these goals such as design tolerances, materials used, etc. We do not recommend the purchase of a beginning level instrument for middle school students because of these limitations.

Intermediate
Instruments in the intermediate category are designed with closer tolerances than beginning level instruments and use better materials. This translates to better sound and intonation as well as easier playing. We recommend that middle school level students begin their search for a new instrument within this category.

Professional
Professional instruments represent at or near the state of the art in instrument design, manufacture and material use. We recommend that you consult a professional musician (such as a private teacher or the band director) before making a decision to purchase an instrument in this category.

Finally, every student at the middle school level should own their own instrument. Students that own their own instruments take better care of then, play better and have more fun with music.