A galaxy is a collection of stars, gas, and dust bound together by gravity. The smallest galaxies may contain only a few hundred thousand stars, while the largest may have hundreds of billions. Galaxies are spread about by vast distances, separated by (relatively) empty space. Their diameters may range from hundreds of light years across (for the smallest dwarf galaxies) to hundreds of thousands of light years across.
Here is Part 1 of a collection of some of my favourite images of galaxies. All credits go the respective owners of these images, and the Astronomy Picture of the Day, where these can all be found with detailed explanations.
The Sombrero Galaxy:
The Andromeda Galaxy:
The Milky Way Galaxy: