Galaxies are enormous groups of stars, stellar remnants, dust and other structures. Not counting galaxy clusters and other large-scale cosmic structures, galaxies are the largest and most massive objects in the Universe, ranging from just a few hundred parsecs in diameter for the smallest and 100 kiloparsecs for the largest.
Probably the most important parameter of a galaxy is it's type. Spiral and barred spiral galaxies appear to be the most common type of galaxy, elliptical galaxies coming in as a close second and irregular galaxies being the most uncommon. This classification determines the stellar composition of the galaxy, as well as the frequency of some types of nebulae and clusters.