If you look closely at the background sky, you can also see some other smaller galaxies (in this picture).
M51 is two interacting galaxies as can be seen in the picture. The larger spiral galaxy is also known as NGC5194, and the smaller irregular galaxy is also known as NGC5195. The common name for M51 is the Whirlpool Galaxy and is about 30 million light years away. This means that the light in this picture started to travel towards us 30 million years ago! It is absolutely staggering when you start to think about this.
The centre of these galaxies are thought to contain very large black holes. They do not look black because they are surrounded by huge amounts of material in which star formation is occurring very rapidly. This material is very dense, very active and very bright.
Although the spiral arms make it look as though the material is unevenly distributed within the galaxy and spinning like water down a plug hole, this is thought to not actually be the case. The material is thought to be evenly distributed across the plane of the disc, what you are seeing in the picture are waves of star formation created by gravitational waves moving though the stellar medium. The material in the galactic disc is orbiting the central black hole, the spiral appearance is partly because material closer to the centre is moving faster, and partly because it is all moving in orbits that are slightly differently aligned. This has an effect like cars on a motorway, where they sometimes seem to bunch up for no apparent reason, then a few miles on the road seems clear again. From a helicopter above you would see a density wave moving though the traffic a bit like star formation in a galaxy.