The choicedropdown form field creates a dropdown list with multiple options. It is different from a standard select dropdown in that each option can have extra icons and descriptions. This field is often used in forms to facilitate the selection of a non-trivial option that demands additional information.
Moodle's Form API is an extension of the Pear HTMLQuickForm API, which is no longer supported. Some documentation for the upstream library is available in the PEAR package page, including a short tutorial. A longer tutorial is also available, courtesy of the Internet Archive.
Form are created using the Form API. The Form API supports most standard HTML elements, including checkboxes, radio buttons, text boxes, and so on, adding additional accessibility and security features to them.