Row Source is typically used to determine how to build a list of items whereas Control Source determines what field will be used to store or retrieve the value. For example, in a Combo Box you have both properties. The Row Source determines how to build the list the user sees when they hit the down arrow. The Control Source determines where to store the value that the user selects.
EXAMPLE Suppose we have a form that is bound to table called Cars which lists information about automobiles. One of the columns in this table is used to store the color of the car. Let's suppose that column is named BodyColor. You also have another table of allowed colors (e.g. Blue, Yellow, Green, Steel Blue, Midnight Blue etc.). You want to ensure that users choose from this list of colors when they enter a value for the car's color.
On our form we add a Combo Box where we set the following properties:
Control Source : BodyColor
Row Source : SELECT Colors.Name FROM Colors ORDER BY Colors.Name;
Row Source Type : Table/Query
When a user sees your Combo Box, they will be able to hit the down arrow on the Combo Box and see a list of colors. When they choose a color, the Form will save their selection into the BodyColor column.