Another Theory of Constraints post: Henrik MÃ¥rtensson has posted his Chain Theory Webcast to YouTube. This is a good approach to the basics of TOC. I screen-grabbed this image from the video that shows a process with work piling up inside.
The idea of a Chain is a helpful metaphor: the chain (system) is only as strong as its weakest link. And the total chain (system) is the collective weight of all the links.