The Andromeda Galaxy is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from the Earth and the nearest major galaxy to the Milky Way. Its name stems from the area of the sky in which it appears, the constellation of Andromeda. Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies (clouds of gas).

It is one of a few galaxies that can be seen unaided from the Earth. In approximately 4.5 billion years the Andromeda Galaxy and the Milky Way are expected to collide, and the result will be a giant elliptical galaxy. Andromeda is accompanied by 14 dwarf galaxies.

More Info: en.wikipedia.org