Telerik RadComboBox and RadDropDownList look much the same
especially when it comes to the dropdownlist and single selection functionality. The RadDropDownList is used in a scenario where the drop-down has to accomodate large sets of data whereas RadComboBox can be simply explained as a textbox with advanced features such as auto completion, filtering functionality, multiple selection support and Checkbox support.
The exact difference between these controls is properly explained in the Telerik documentation. Please have a look in the following documentation