This guide is good at going in depth without being too heavy on the jargon. I've found that it's still a bit much for younger students learning about the concept and related math for the first time. George Gillard's intro to PID [1] has been a staple of VEX robotics instruction because it fills this niche -- it's an excellent resource for teaching high school students and younger.

1. http://georgegillard.com/documents/2-introduction-to-pid-con...