|MadSci Network: Earth Sciences|
Snowflakes, and indeed all snow crystals, are six-sided due to the geometry of the oxygen and hydrogen atoms that form the crystals. Different temperatures and different pressures favor minor differences in the lattice structure formed which determines the shape of the resulting crystal. For the nitty gritty details, check out this website from CalTech.
Start with this USATODAY page on snow crystals and follow the links to images and more information about all kinds of crystals and snow flakes.
There are a couple of books about Wilson Bentley (from the Jericho VT Historical Society) you might want to check out. Wilson Bentley was a Vermont farmer who was fascinated by snowflakes and over his lifetime took thousands of pictures of them and contributed a tremendous amount to the body of knowledge about snow crystals. Both books are available in bookstores and libraries. Snowflake Bentley by Jacqueline Briggs Martin, is for younger readers, while The Snowflake Man:A Biography of Wilson A. Bentley by Duncan Blanchard will fascinate anyone interested in weather and snow.
Try the links in the MadSci Library for more information on Earth Sciences.