The Circle Selector gives you a circular cursor to use in selecting vertices, edges, faces, and objects. The selection with the circle selector is cumulative. So, using it adds to the vertices that have been selected:
Press the A key once or twice to deselect everything.
Press the C key. Notice the circle around the cursor. Tap the LMB over the vertices to select them. You can also hold down the LMB and move the mouse to select more vertices.
To stop the selection, use either the RMB or the Esc key.
Press the C key again. Hold down the MMB while moving over the vertices selected earlier.