What is a select box /dropdown menu?

A select box, may also be viewed as a "dropdown menu". 

1.    A dropdown menu has a similar function to radio buttons, in that only one option can be chosen. 

2.    On the other hand a dropdown menu gives the customer the option to not choose a product. 

3.    Dropdown menus are suitable when there are many different choices.

Another example would be selecting dessert from many possible choices (five or more) including not having dessert at all.