Galaxies are collections of stars. We are in the Milky Way Galaxy. Many scientists believe there are black holes at the centers of galaxies, and that these objects hold the galaxies together. This is controversial, so you might want to do some research on what scientists currently believe.

A galaxy is a collection of about 100 billion (that's 10¹¹, or a 1 with 11 zeros) stars, held together by gravity.

A galaxy is an enormous swarm of a million to a trillion stars. The Milky Way Galaxy,(our galaxy) is medium-large, with 100 billion stars. There are about 100 billion galaxies in the known universe, so there are literally as many suns as there are sand grains on all the beaches of Earth. The nearest large galaxy (Andromeda) is 2 million light years away. When you look out into space, you are also looking back in time. You actually see Andromeda as it was 2 million years ago, because it took that long for the light to reach Earth.

Stars like to clump together. Each clump is a galaxy. It turns out that even galaxies clump together. And that clumps of galaxies, clump together. In fact, there is a very large scale structure to the universe that is definitely not well understood at all.

What holds galaxies together? Gravity. All of the stars of a galaxy are all attracted to each other. This attraction is weak because the stars are very far apart, but it is enough to keep the galaxies together. There is also a large amount of matter that has never been seen (called "dark matter") that helps keep galaxies together.

A galaxy is collection of stars, gas and apparent dark matter (which is detectable by its influence on the orbits of stars, but in otherwise not understood). Galaxies are bound in the sense that the stars cannot leave the galaxy for the most part, and orbit the center of mass of the galaxy. (The sun orbits the Milky Way in about 200 Million years). Nearly all stars in the universe are bound into some galaxy.