There are several options... and all of them are shown in the DAW samples as garret said.

dbComboForm: provdes a list of choices based in a field validation object defined for the column in your DD.

Example: oCustomer_State object inside Customer.vw view in the OrderEntry sample worskpace.
look there, there is noting defined at the object level. So, where the list of states come from ? Look at the Cutomer.dd there is a validation table defined for this column.

The validation tables could be of 3 different types. Simple list, (fixed hardcoded values) , CodeValidationTable, (values registered in special tables called CodeMast/CodeType
And FileValidationTable, which can be derived from tables that exists in your system..

All of this are better explained in the help. so please take a look at

There are other type of combo, that provides values based in a parent table.
dbParentCombo (

Example: OrderEntry workspace, DemoParentCombo.vw view, oOrderHeader_Customer_Number obejct.

Pretty useful, where the number of parent records are not too big.. You dont want a list of 5.000 different values in a combo list.. For bigger volumes, there is the Prompt/Selection Lists,, also shown in the examples..