Galaxies are by definition vast rotating systems of stars, gas and dust. They are the building blocks of the universe. Galaxies range in size from dwarf types, only a few thousand light years across to great spiral and elliptical galaxies spanning several hundred thousand light years across. They contain anywhere from a few million to as many as one trillion stars.
The earth is located within a large spiral arm (the Orion arm) in a spiral galaxy we call the “Milky Way”. The closest galaxy similar to our own is the Andromeda Galaxy (M31).
[description from Rob Gendler’s primer]