Raspbian is based on Debian.
Debian generates three releases: stable, testing, unstable.
The "stable" distribution contains the latest officially released distribution of Debian.
The "testing" distribution contains packages that haven't been accepted into a "stable" release yet, but they are in the queue for that. The main advantage of using this distribution is that it has more recent versions of software.
The "unstable" distribution is where active development of Debian occurs. Generally, this distribution is run by developers and those who like to live on the edge.
Raspbian sort of tracks stable and testing. It does not have the equivalent of unstable.
jessie is the current testing. If jessie doesn't use the version you want you'll need to wait or build it from source.