Edit → Select Mode (S or E S) switches on the selection mode. All unfinished actions are rejected as well as all unfinished object changes. If you just started a primitive object creation, it will be rejected as well. In the mode you may select object by clicking at them using the left mouse button, or by selection box. Selected objects are always rendered in the selection color so you can easily distinguish them from all other objects. You cannot select locked objects this way. They can be selected by selection box or Edit → Select Locked (E K). Holding Ctrl will toggle the selection state of the objects you are clicking at or selecting by selection box, while holding Shift in the same situation will always add objects to the selection.