Galaxies were first systematically classified, based on their visual appearance, by the famous astronomer Edwin P. Hubble in the late 1920s and 30s, during years of painstaking observations. Hubble's Classification of Galaxies, as it is known, is still very much in use today, although, since Hubble's time, like any good classification system it has been updated and amended in the light of new observations.
Before Hubble's study of galaxies, it was believed that our galaxy was the only one in the universe
Astronomers thought that the smudges of light they saw in their telescopes were in fact nebulae within our own galaxy and not, as Hubble discovered, galaxies in their own right. It was Hubble who demonstrated, by measuring their velocities, that they lie at great distances from us, millions of light-years beyond the Milky Way, distances so huge that they appear tiny in all but the largest telescopes. Moreover, he demonstrated that, wherever he looked, galaxies are receding from us in all directions, and the further away they are, the faster they are receding. Hubble had discovered that the universe is expanding.