What exacly is the so-called "formless" jhana?

When reading the suttas, sometimes I come across to the expressions: "dimension of the infinitude of space", "dimension of the infinitude of consciousness", "dimension of nothingness" and "dimension of neither perception nor non-perception". What exactly is the meaning of the expressions above? Why they are "formless"? When the Buddha describes Right Concentration, he mentions only the four "form" jhana, why he excluded the "formless" jhana as Right Concentration?