The center colours, created by optical interference between multiple layers of air and ice, are the main attraction. You'll also notice that this snowflake is quite bumpy towards the outer tips - this is "rime", formed when the snowflake encounters larger water droplets that freeze on contact.
More science: you'll notice a hexagon just 9-o-clock of center, hiding in the background. This is probably the center on another snowflake beneath the surface, but I can't see more of it's structure to say why or how it got there. The lighter areas of the snowflake are reflecting more light based on the angle of the flash and an extra layer of ice helping with the reflecting. Finally, look super-closely at the upper-left branch and you'll see an interference pattern switching from purple to green bands. Want to see it better? click here for a higher-res version: http://donkom.ca/gplus/DKP_6599.jpg