In between the constellations Cassiopeia and Pegasus like an innocent looking blur in the sky. The blur is the Andromeda Galaxy or M31, it's the most distant object that we can still see with the naked eye. Probably because it's such a large galaxy. That innocent looking blur in the sky is looking on our galaxy, the Milky Way, with hungry eyes.
Like so many galaxies, Andromeda is very prone to galactic cannibalism. Galactic cannibalism is when two galaxies pass too near to each other and the galaxy with the stronger gravitational force strips the other of it's planets and "eats" it. Of course, Andromeda isn't the only galaxy that likes to eat it's neighbors, evidence shows that even the Milky Way has done it in the past.
Most of the objects around our galaxy are redshifted, which means they are moving away from us. However, astrophysicists have found that the Andromeda galaxy is blueshifted, which means it is coming at us.
It's believed that galactic cannibalism is quite a common occurence in the universe, but we have such a limited scope of the universe that there is no exact way to measure how often. It's just the nature of the universe that big galaxies get bigger and smaller ones are eventually absorbed. It seems that the universe is still settling after the events of the big bang.
This may sound like the end of life for us, but unless you plan for living for billions of years there's no reason to fear. The fight between Andromeda and the Milky Way is about 4.5 billion years away. 
By then, the Earth very well may have exhausted all the life on it anyway.
They estimate that in about two billion years the process will start when our galaxies float past each other. A tail of stars will become entangled around Andromedas' tail and we'll absorb into each other. The Andromeda galaxy is home to trillions of stars and evidence suggests that it's absorbed several other galaxies in the past, but the exact number is unknown. It's possible when our two galaxies combine, that no catastrophic event will happen, merely that the night sky will change drastically. The trillions of stars in the Andromeda galaxy are spaced about as big as two grains of sand separated by a football field, so it is likely that our solar system will just find a small empty space to make home without any damaging collisions.
Though a collision is only one of the worries, the more pressing worry for life on Earth in those billions of years is where the sun will be. If it ends up too far, we'll freeze, too close, we'll boil. Or of course the sun could become obscured by one of the many clouds of gas and dust that gives every galaxy it's color. In the end, if nothing major happens, it will give humans a vastly better range on the universe, a closer look at stars that were previously unexaminable or undiscovered, and a brilliant new night sky.