Hi Sam,
Selection Screen is used to obtain input form the users. The Selection Screen is generated only when there is a PARAMETERS statement in the report.
The Screens are used for displaying the output to the user such as list of information.
Example:
Suppose you want to display the states belonging to a particular country, then in the selection screen you provide a dropdown list for selecting the country that the user is interested in. After selecting the country, on executing, on the next screen (which is also called as output screen) the states belonging to selected country will be populated along with State ID, State Name, etc in the list format.