In the ground propagation, the radio waves travel through the lowest portion of atmosphere, hugging the earth. These very low frequency signals emanate in all directions from transmitting antenna and follow the curvature of planet. In sky propagation, the higher frequency radio waves radiate upward into the ionosphere, where they are reflected back to the earth.
In the line of sight propagation, very high frequency signals are transmitted in straight lines directly from the antenna to antenna. Antennas must be directional, facing each other and either tall enough or close enough together not to be affected by curvature of the earth. The line of sight propagation is tricky as radio transmissions can not be completely focused. Infra red waves are used for the short range communication such as those between a PC and the peripheral device.