When STK was first released, it came with a little html primer that was a really well explained introduction to orbits and spaceflight dynamics. It’s now available on AGI’s website, although it’s a little buried within the educational section of the website.

As the site says:

Based on the book Understanding Space: An Introduction to Astronautics Copyright
1996 McGraw-Hill Inc. by Jerry Jon Sellers, Wiley J. Larson (editor), this brief astronautics primer is designed to provide the reader with a conceptual overview of important topics in orbital mechanics. Understanding these key concepts will enhance your insight into the science behind Satellite Tool Kit and better equip you to apply these concepts to practical problems in space.”

If you are interested in learning more about space – in a way that anyone can understand – I heartily recommend you click on this link:

http://www.agi.com/partners/edu/AstroPrimer/primer1.htm