The border selection works similar to the Marquee tool in Flash or Photoshop, stretching a border around the vertices you want to select:
Press the A key, once or twice, so that everything is deselected.
Move your cursor above and to the left of the cube.
Press the B key. Notice that the cursor has become dotted—crossed lines as on the left-hand side of the previous illustration.
Hold down theLMB while you drag the cursor down and to the right. Any vertices within the border will be selected when you release theLMB. Just select the vertices on the left side.
Press the B key again. Make another selection, but start at the bottom-right, and move to the top.
Now press the B key, move over vertices you have already selected, and press theMMB as you drag the cursor.