I'm writing an article about blue screen effects in movies and I
wondered if
you could help me understand the process.

What I need to know is:
- What a blue screen is and its purpose
- What's involved with shooting live action against a blue screen
- Most importantly, an explanation _how_ the CG graphics are created
and _why_ it takes so long to create just a small amount of finished
footage.

Any info or insights you can share, or links to information sources,
is much appreciated.