What is convention vs configuration?

Answer 1

Someone can probably give you a textbook definition, but until they do here is a real world example.

In asp.net mvc there are views and controllers:

• Configuration - would have been manually mapping controllers to use specific views, almost like a codebehind scenario.
• Convention - A Controller method named Index most likely wants to use a View called the same, so the framework goes and looks for that.

Basically don't explicitly define all correlations in code, but let frameworks/code make educated guesses about what should be done based on predetermined patterns.