On his blog, phpcult.com, Vidyut Luther talks about the difference between require and include in PHP.

This should be well known, and people should be aware as to why they are using either or. But, I've noticed lately that a lot of people new to PHP or programming are not aware of the difference. Depending on what you need you, need to decide what the differences are.

It's a short post, but for someone just getting into PHP, it's some handy information to have. He looks at both functions and gives what they're good for. He does throw in a few caveats, though - including an issue with parse errors in included files pre-PHP 4.3.5...