Galaxies come in three main types: elliptical, spirals, and irregulars. A slightly more extensive description of galaxy types based on their look is given by the Hubble sequence. Since the Hubble sequence is completely based upon visual morphological type, it may miss certain significant individuality of galaxies such as star formation rate or activity in the core.
Our own galaxy, the Milky Way, occasionally simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kilo parsecs or a hundred light millennia in diameter and three light millennia in thickness. It contains about 3×1011 (three hundred billion) stars and has a whole mass of about 6×1011 (six hundred billion) times the mass of Sol.
In spiral galaxies, the spiral arms have the contour of approximate logarithmic spirals, a pattern that can be notionally shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms also revolve around the center, but they do so with steady angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are consideration to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are noticeable because the high density facilitates star formation and they therefore port many bright and young stars.
Despite the fame of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. These tiny galaxies are about one hundred times smaller than the Milky Way, holding only a few billion stars. Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites. Dwarf galaxies may also be classified as elliptical, spiral or uneven. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.