Graceful degradation is a design philosophy that centers around trying to build a modern web site/application that will work in the newest browsers, but fall back to an experience that while not as good still delivers essential content and functionality in older browsers.
It is a useful technique that allows Web developers to focus on developing the best possible websites, given that those websites are accessed by multiple unknown user-agents. Progressive enhancement is related but different — often seen as going in the opposite direction to graceful degredation. In reality both approaches are valid and can often complement one another.
- Graceful degradation on Wikipedia