Pressing the A key to select all the vertices is the easiest selection to do. It selects all the vertices, or deselects them if they are all selected. It's often used to clear the current selection, so that you can choose a different set of vertices:
Pressing the A key either selects or deselects everything. You worked with vertices, but this applies to edges, faces, and objects as well. If you look just above the 3D View window on the right as you do it, Blender shows you how many vertices you have selected.