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A Definition
A spiral galaxy is a collection of stars with a bright, large center and arms that appear in a swirl fashion around that center. The arms contain younger stars, while the stars closer to the center are much older stars. The composition of the bright center is highly theoretical and is said to contain a black hole. The spiral designation is one of the three main classes of galaxies described by Edwin Hubble in his work, "The Realm of the Nebulae." The other two classes of galaxies are elliptical and lenticular. This is but one of several different classifications of galaxies based on a number of variables.
Closer to Home
Our galaxy, the Milky Way, is a spiral galaxy, and the Earth is in one of the arms swung out from the center. Our galaxy is approximately 100,000 light years wide, and weighs approximately one trillion solar masses. This is the basic make-up of a spiral galaxy. Most spirals are elongated and some have a tighter spin. The largest collection of spiral and other galaxies in the night sky is in the constellation Virgo. There are several hundred galaxies, most of them spiral. The total number of galaxies is not known, but it is estimated to be in the trillions.
In the Middle
The center of our galaxy sits about 8 kilo parsecs away from us. It is surrounded by tens of thousands of stars, and houses a super-massive black hole at its geometric center. That black hole gives astrophysicists quite a laboratory for the research of black holes, star clusters, star creation and star destruction within the gravitation rules of a spiral galaxy. This research can then be used when studying spiral galaxies that are further out from us to see if there is, indeed, a pattern in their composition.
Dark Matter
Other research performed on spiral galaxies includes the concept of the "gravitational glue" that keeps all those stars and all that gas together in one shape. This glue, known as Dark Matter, is believed to be a solid substance devoid of light reflection, yet has mass and takes up space. This substance would produce the same effect in a given field (either magnetic or gravitation) that a product would that generates or reflects light. There is research that is unclear at this time concerning dark matter's existence.
The Future
Another spiral galaxy, the Andromeda Galaxy in the constellation Cassiopeia, is a galaxy that is often considered by astronomers to be a sister galaxy to our own. It gives us an opportunity to see how our galaxy functions over long periods of time.