A drop-down list is a field which provides a list of pre-defined options. A user clicks a down arrow at the end of the field to display the list of options. There are two types of drop-down lists:
Standard drop-down list: You select from a list of pre-defined values. Only a developer or administrator can modify this list of possible values.
Data Combo drop-down list: You select from a list of records in a related service. The available options change when new records are added to the related service.