Galaxies are massive groups of stars and gas that are spread throughout the universe. Our solar system is located within the Milky Way galaxy. We can only see a part of the Milky Way galaxy, because it is huge. It would take millions of light years to cross it.
Galaxies were known, but they were thought to be small, until Edwin Hubble discovered that galaxies were giant in 1924.
There are different types of galaxies. There are Spiral, Barred Spiral, Elliptical and Irregular galaxies. Spiral galaxies are divided into three categories: A,B and C. A spiral galaxies have many flat, tucked arms. B galaxies have fewer arms, but they are still tucked. C galaxies have a small amount of loose, un-tucked arms.
Barred Spiral galaxies are essentially the same as the Spiral galaxies, with the same categories, but they have a bar in the middle instead of a circle.
Elliptical galaxies range from round, with no arms, to oval, with no arms. Irregular galaxies are all shapes and sizes, and are made up of mostly gas and dust.
Galaxies are gathered into clusters. Clusters are massive groups of galaxies. Super-clusters are clusters of galaxy groups, and sheets are gigantically massive clusters of super-clusters. It would take at least a billion light years to cross.
Galaxies are giant spheres of light which slowly move across the unlimited universe. To know all of them, we would have to travel infinitely across the universe.