Matthew McGiffen has a primer for us:
Statistics are vitally important in allowing SQL Server to find the most efficient way to execute your queries. In this post we learn more about them, what they are and how they are used.
Read on for plenty of information about stats. One thing I’d emphasize in this is that, for the most part, auto-generated stats are fine. The only time I’ve ever seen any value in creating my own stats is in multi-column statistics (which auto-generated stats doesn’t do), and even that’s of fairly limited value, at least in my experience. I’m sure that there’s a point in which hand-crafting your own statistics makes a tiny but noticeable marginal difference, but the systems I’ve worked with tend not to be anywhere near that point. There are usually much bigger fish to fry.