== Don't repeat yourself ==
To help to maintain clean code, Rails follows the idea of DRY, an acronym for Don't Repeat Yourself. The idea behind it is simple: whenever possible, re-use as much code as possible rather than duplicating similar code in multiple places. This reduces errors, keeps your code clean and enforces the principle of writing code once and then reusing it. This also helps lead to an API driven structure whereby internal methods are hidden and changes are achieved through passing parameters in an API fashion.
For more information on DRY look at the Wikipedia article