The DropDownList Web server control enables users to select from a single-selection drop-down list box. The DropDownList control is similar to the ListBox Web server control. It differs in that it shows only the selected item in a box.
To specify the items that you want to appear in the DropDownList control, place a ListItem object for each entry between the opening and closing tags of the DropDownList control.
The DropDownList control also supports data binding. To bind the control to a data source, create a data source, such as a ArrayList object, that contains the items to display in the control. Then, use the Control.DataBind method to bind the data source to the DropDownList control.
Use Its SelectedIndex property to programmatically determine the selected item index , and use its SelectedValue to get the value of the SelectedItem.
DropDownList How To :
How to: Add DropDownList Web Server Controls to a Web Forms Page
How to: Add Items in List Web Server Controls
How to: Determine the Selection in List Web Server Controls
How to: Populate List Web Server Controls from a Data Source
How to: Respond to Changes in List Web Server Controls
How to: Set the Selection in List Web Server Controls