The ionizing action of the sun's radiation on the earth's upper atmosphere produces free electrons. Above about 60km the number of these free electrons is sufficient to affect the propagation of electromagnetic waves. This "ionized" region of the atmosphere is a plasma and is referred to as the ionosphere.
Longer wavelength radio signals can be "bounced" off the ionosphere allowing radio communication "over the horizon". This is how the long, medium and short wave radio broadcasts reach receivers over long distances. Because the ionosphere is not a nice smooth "mirror" the signal can be scattered in many directions causing loss of signal strength and interference from other transmitters. The ionosphere is particulary disturbed in the auroral regions, and during magnetic sub-storms.
Shorter wavelength radio signals pass through the ionosphere but are affected by it. These shorter wavelengths are used by satellites for imaging the earth, and the ionosphere affects the images rather like the way the atmosphere causes "twinkling" of the stars.
