When to use a Child Theme
Question: When should you use a child theme with WordPress?
Answer: All the time.
Child themes are great because instead of modifying theme files, you can create your own page templates and custom CSS in a separate place. The reason for doing this is so you can update your theme in the future and not lose your changes.
How to create a Child Theme
This was a great talk by Lauren Pittenger I heard at WordCamp Scranton last year.