Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments.
I bought my board from oomlout. Their kit is full of bits to play with, and I'll probably head back there again! They believe in sharing too. All of their diagrams and photos etc are available on-line. They have also uploaded videos and pictures to instructables.com to help you get the most from their kit.
I've got the hang of the basics, and now I'm starating to design my own circuits. I may publish my efforts here.