Attempting to explain blending modes in a single post is a pretty tall order, as it's a massive and involved subject - in fact, whole books have been written just about blending modes:
Amazon.com: Photoshop Blending Modes Cookbook for Digital Photographers : 49 Easy-to-Follow Recipes to Fix Problem Photos and Create Amazing Effects (Cookbooks (O'Reilly)): Books: John Beardsworth
Perhaps if you could give us a bit more detail about what you're doing, or wanting to do, we might be able to offer more specific advice.