When creating user interface we rely on a common set of controls to solve common problems. Hopefully, the controls are common enough to use again and again and they provide a consistant interface for interaction with our web applications.  

A pattern describes an optimal solution to a common problem within a specific context.

From the IAWiki:

Patterns are optimal solutions to common problems. As common problems are tossed around a community and are resolved, common solutions often spontaneously emerge. Eventually, the best of these rise above the din and self-identify and become refined until they reach the status of a Design Pattern.

Each pattern has four primary components:

1. a title
2. a problem
3. a context
4. a solution

Yahoo! has created a common pattern library where they give visual examples of each pattern.