Have you ever gotten annoyed at how Debian Stable always seems to be out of date? Read here how to cure that 'problem'.
It's all here
Why do this? Stable is covered by the Security Team. Testing and Unstable are not. For non-critical services, like perhaps your mailer, or your window manager, this is not so important, and the newest versions may have additional features that are desired. It is these packages that are perfect for pinning to a version, other than Stable.