I can think of at least five times where a client has pulled me aside in the past year to ask how they can use their camera better, outside of the automatic setting - I've drawn out countless exposure triangles on napkins, newspapers, even an iPad drawing app, and despite my best efforts I can sometimes tell they're still confused from my brief lesson. For now on, no more triangles, I've found a great website that anyone can learn from.
That website is http://www.canonoutsideofauto.ca. No they didn't pay me to say that, less than 20 minutes ago I didn't even know they existed! "Canon Outside of Auto" provides a well-designed interactive interface (based on the Canon exposure setting system) along with clear and easy-to-follow instructions, allowing the user to visualize how each adjustment they make effects the final photo.
What I really like about the site, is it allows the user to decide how far they want to go with their lesson, all within 3 pages. The "Learn" page describes each setting/principal and offers a visual of the explanation for aperture, shutter, ISO, and exposure. The user can then move on to the "Play" page, where they can apply what they learned using sliders to create a photo of an example scene, giving the freedom to make creative decisions on motion blur, aperture, etc. Or use aperture/shutter priority modes. Once the user is comfortable with the concept, they can then move on to the "Challenge" section of the site, which provides a series of timed tests with instructions for certain exposures, motion blur/still, foreground/background blur, etc. Making sure that they have a full understanding of how each setting works. If at any time someone is confused by the lingo, they also include a full glossary with diagrams, further summarized explanations in "Photography 101", and helpful tips for real world scenarios.
This is far better than anything I could possibly provide with some words and doodles, and I can't imagine it could get much simpler than how they lay it out. Being a Canon shooter I'm not sure if there's anything equivalent for other brands (I searched, but feel free to correct me in the comments), but besides some different terminology, it all translates.
If you know anyone trying to learn photography I urge you to link them to this, and let me know any feedback they have in the comments below!