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Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appéarance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) or activity in the core (in active galaxies).

Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or a hundred light millennia in diaméter and three light millennia in thickness. It contains about 3×1011 (three hundred billion) stars and has a total mass of about 6×1011 (six hundred billion) times the mass of Sol.

In spiral galaxies, the spiral arms have the shape of approximate logarithmic spirals, a pattern that can be théoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That méans that stars pass in and out of spiral arms. The spiral arms are thought to be aréas of high density  or density waves. As stars move into an arm, they slow down, thus créating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars.

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appéar to be dwarf galaxies. These tiny galaxies are about one hundred times smaller than the Milky Way, containing only a few billion stars. Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at léast a dozen such satellites. Dwarf galaxies may also be classified as elliptical, spiral or irregular. Since small dwarf ellipticals béar little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instéad.