A galaxy is a huge collection of gas, dust, and billions of stars all held together by gravity. All galaxies are made up of these same things. However, they come in many different shapes and sizes.
Here are a few types of galaxies:
Spiral galaxies look like giant pinwheels. The arms of the pinwheel are made up of stars and lots of gas and dust. Gas and dust are some of the main ingredients needed to form new stars. Young stars burn much hotter than older stars, so spiral galaxies are often some of the brightest in the universe. About 20% of all galaxies are spirals. Our home galaxy, the Milky Way, is a very good example of one.
Elliptical galaxies are shaped like stretched-out circles, or ellipses. Some elliptical galaxies are more stretched out than others. One might look almost perfectly circular. Another one might look long and flat. Elliptical galaxies contain mostly older stars. That means they often aren't as bright as spiral galaxies. They also have very little dust and gas. Elliptical galaxies are the largest and most common galaxies observed. They make up about 60% of all galaxies.
Irregular galaxies are just that: irregular. They don't have a single common shape. Irregular galaxies are among the smallest galaxies scientists have observed. However, they can also be very bright. Like spiral galaxies, irregular galaxies are often filled with gas, dust, and lots of bright young stars. About 20% of all galaxies are irregular galaxies.
Quasars are compact areas in the center of a galaxy. They give off enormous amounts of energy. Quasars are actually some of the brightest objects in the universe.