The short version
You can safely install the PostgreSQL package from the backports. You'll probably get a stable piece of software that will not nuke your system.
The longer version
Debian comes in three flavours: stable, testing and unstable. See Debian Releases. You are apparently running Debian stable, which is the preferred release for a production environment.
Since you claim that you need PostgreSQL in version 9.1, you can check the package repository and you'll find out that the PostgreSQL version for the stable release is 8.4, for the testing and unstable release 9.1
What you can do is to change your Debian version from stable to testing. However, another option instead of updating the entire system to another version, is to use a back port. This is a single or a few (if it's more than a few you should consider upgrading to testing) packages from testing or unstable compiled for the stable release. The benefits are that you can stay on the Debian stable branch and use the more up-to-date software that you need (here: PostgreSQL). The software has not been extensively tested, otherwise it would be in stable already, but it's unlikely that this package will break your system. This is a more safe approach than to change to the testing or unstable branch.