We are able to see objects because our eyes detect the light emitted or reflected by objects. Light consists of electromagnetic waves. The different colors of light are electromagnetic waves of different lengths.
Visible light, however, covers only a small part of the range of wavelengths in which electromagnetic waves can be produced. Radio waves are electromagnetic waves of much greater wavelength than those of light.
For centuries, astronomers learned about the sky by studying the light coming from astronomical objects, first by simply looking at the objects and later by making photographs. Many astronomical objects emit radio waves as well as light waves but this fact was not discovered until 1932. Since then, astronomers have developed sophisticated systems such as the VLBA that allows them to make pictures from the radio waves emitted by astronomical objects.
A number of celestial objects emit more strongly at radio wavelengths than at those of light so radio astronomy has revealed many surprises in the last half-century. By studying the sky with both radio and optical telescopes, astronomers can gain a much more complete understanding of the processes at work in the universe.