No website dedicated to keeping these "tape time-machines" running would be complete without an overview of magnetic recording.
Rather than try and re-invent the "reel" here are some links to sites that, from my professional judgement, explain the principle well:
Reverb.com - a professional audio equipment auction site.
Wikipedia - were would we be without a Wiki page!
How Stuff Works - stuff like tape recording.
Hyper Physics - this article is a bit more scientific but still has simple diagrams.