RadGridView allows the user to select a cell or a row. By default this functionality is disabled and to turn it on you have to set the SelectionUnit property to Mixed.
In order to select a full row when the SelectionUnit is Mixed, you can click somewhere on it outside of its cells.
RadGridView provides three selection modes, which allow you to manipulate the type of selection. The following bullets describe the results when they are combined with SelectionUnit - Mixed:
SelectionMode - Single: You can select only one unit at a time.
SelectionMode - Multiple: You are able to select any cell and any row. Units are added to the selection when they are clicked and get removed when they are clicked again.
SelectionMode - Extended: Units are added to the selection only by combining the mouse clicks with the Ctrl or Shift key.
For more information take a look at the Basic Selection topic.