Rows in RadGridView can be selected by a translucent rectangle selection. You should set the MultiSelect property to true and this will automatically enable the rectangle selection. The user just needs to perform a mouse drag operation on RadGridView. A translucent rectangle will be created to indicate the possible selection of rows or cells.
Use the ShowTranslucentSelectionRecatangle of the GridTableElement to enable or disable the selection rectangle:
Copy[C#] Disabling the selection rectangle
((GridTableElement)this.radGridView1.TableElement).ShowTranslucentSelectionRectangle = false;
Copy[VB.NET] Disabling the selection rectangle
DirectCast(Me.RadGridView1.TableElement, GridTableElement).ShowTranslucentSelectionRectangle = False