Other stars may have solar systems, too. A few planets around other stars have been discovered. Click here to learn more about these extra-solar planets. A galaxy is a gravitationally bound collection of stars, gas and dust orbiting a common center. The band of light circling our sky and visible from dark sites, known as the Milky Way, is the light of millions of faint stars in the plane of our galaxy.
Galaxies typically have billions of stars.
They often cluster together and sometimes collide and merge. Check out the archives of the Astronomy Picture of the Day for nice views of galaxies. The collection of all the galaxies and the space and time they inhabit. Everything there is that we can see or detect in any way, either directly or by inference, is part of the Universe. It is the largest structure there is, by definition.