If you use a camera, phone or mp3 player that takes SD memory cards, you'll probably find this web site useful. The storage form is evolving, and there are more types of cards available all the time. Here's where you can sort it all out:
And a good article to help if you're still confused:
No, I don't write this stuff. I share what I find when I stumble across it and wish I'd found it sooner!