If you are learning photography, want a brush-up, or are just plain curious, I heartily recommend bookmarking the following links. They come highly recommended, and not just by me!
Jodie Coston: Online Photography Course — The best free on-line course I've found!
Geoff Lawrence: Photography Tutorials
Gary Voth: Articles and Tutorials
Steve Hoffmann: Tech Articles and Tutorials
Matthew Cole: A Tedious Explanation of the f/stop — Everything you'll ever need to know!
Matthew Cole: A tedious Explanation of Depth of Field — DoF from soup to nuts!
Matthew Cole: Lightning Photography — How to capture the flash!
NK Guy: Flash Photography with Canon EOS Cameras — THE guide to flash photography!
Joe McDonald: Flash Primer Article [In four parts]
Chuck Gardner: A Holistic Approach to Lighting and Digital Photography — Really good stuff!