The Andromeda Galaxy (known as M 31 or NGC 224 in the catalogs), is a giant spyral galaxy about 2,5 mln l.y. away from Earth in the direction of the contellation Andromeda, where it gets its name from. M 31 is the closest big galaxy to us and is visible to te naked eye under dark skies with it being the farthest object visible by a human without the aid of any instrumentation.
M 31 is the biggest galaxy among the ones in the Local Group, a set of galaxies containing the Milky Way, the Triangulum Galaxy and about fifty smaller ones.
In this particular picture you can also see two interesting novae: explosions generated by stars at the end of their life cycle. You can see them in detail in the two attached images!