One of the more useful woody sites on the web is Eric Meier's wood-database.com. Besides having a nice alphabetical listing of several hundred of the most common woods in woodworking, he occasionally posts detailed articles and informative videos.
One of the most common questions I get at shows and by email is "how can I tell if a piece of wood is a hardwood or softwood?" Well, there are a bunch of ways, most of which are way too technical for most folks...but Eric does a great job in the following video of giving one of the simplest answers in a straightforward, yet technically sound wood science way. The secret is in understanding the science of growth rings.
And if there are no growth rings, or you want to be able to distinguish between different hardwoods? Well, then...you sign up for one of Randy Wilkinson's wood identification classes.