Known to the ancients as a "little cloud", the true nature of this great galaxy was not discovered until 1923 when Edwin Hubble, using the 100-inch telescope at Mt Wilson, found that it was simply too far away to be a local cloud of gas and stars. From a study of its cepheid stars, he determined that the "Andromeda Spiral Nebula", as it was then known, had to be at the unheard of distance of some 900,000 light years. Therefore it had to be a separate island universe and not a nearby part of our Milky Way. And so came the idea that the Milky Way was not unique, but just one of many galaxies in our universe.
Today we know that galaxies are innumerable. There are "kazillions of them" -- so say the school kids. But let us not lose sight of the fact that the Great Andromeda Spiral Galaxy, up there and visible to the naked eye on dark Autumn nights, is indeed a monstrous galaxy. It is 20 times more massive and 40 percent larger than our Milky Way, which itself is quite large as galaxies go.
M31 dominates the group of over 35 galaxies in our galactic neighborhood, collectively known as the "Local Group". They are not only nearby but the whole bunch is travelling through space in a coherent, uniform speed and direction. The center of the pack lies about midway between us and M31. There are only three spiral galaxies, like gigantic pinwheels, in the Local Group -- M31 the Andromeda Galaxy, our Milky Way and the dim but famous Triangulum Galaxy (M33). All the others are either smaller "dwarf" elliptical galaxies or irregular galaxies that defy classification. So M31 is the nearby "boss" galaxy. But it may be a small consolation to know that our Milky Way has more galactic companions than M31. We have 11 known companions, with the Magellanic Clouds being the nearest and best known; whereas M31 has but seven, of which M32 and NGC 205 are its nearest and best known.
That little bit of elongated, fuzzy light you see up there is not a little galaxy by any means. M31 has a length of 180,000 light-years (one light year is about six trillion miles), it contains over 300 billion stars, 200 or more O-B associations, 300 or more globular clusters and 400 or more open clusters. It is a very dusty galaxy and probably constantly giving birth to new stars. The first extragalactic supernova was found in M31. Like many of the major galaxies, it has a black hole at its core. But its black hole is of immense mass -- that of 30 million Suns -- and ringed with a torus (a "doughnut") of stars spiraling to their doom. The Great Andromeda Galaxy is indeed great and not at all the "little cloud" of the ancients.
It lies not at a mere 900,000 light-years as surmised by Hubble but at some 2.20 to 2.93 million light-years away; and is often said to be the most distant object readily visible to the naked eye.
For the technophiles, M31 is a very bright, S-shaped, non-barred spiral shining at magnitude 3.4. The NGC description is given as "!!! eeB eL vmE" which translates to: "A magnificent object, most extremely bright, extremely large and very much elongated." It is not face-on but somewhat tilted at 15 degrees. It is in approach (coming towards us) at 35 kps (22 miles per sec). Its dimensions of 185x75 arcminutes make it too big to be seen in its entirety in the narrow fields of view presented by telescopes. The true breadth of M31 is best reckoned in binoculars.
For an excellent deep-sky picture of M31 expertly annotated to show its more prominent features see the picture in Sky & Telescope, Nov 1997, page 108.