Data Binding

RowSource is a table/query for a ComboBox or ListBox.
ControlSource is a field to which a control is bound.

Row Source - It is used to fetch the data to the control for which the property is set.
Control Source - If more than one field/column is retrieved in the Row Source property

The Control Source, as was mentioned by Rural Guy, is the property of a control which is normally set to a field name that exists in the form's Record Source (table, query or select statement). You can type a formula in there too but then it will not update a table. It has to be "bound" to a field from the table, query, or Select statement so that Access knows where to store or edit the data for that control.