Services, a feature introduced in the early days of OSX is unknown to many Mac users. However, learning about Services will take your understanding of OSX to the next level.
Services are a way for applications to communicate with each other. These services allow you to use a set of features from one application while using another. For example, the Mail service in the Finder opens the Mail application so that you can email a file directly from the Finder without opening the Mail application separately. This is a handy way to send file attachments.
In this video from C|Net, OSX Services and the Services Menu are introduced.