Historically MacPython came with a number of useful extension packages included, because most Macintosh users do not have access to a development environment and C compiler. For Mac OS X that bundling is no longer done, but a new mechanism has been made available to allow easy access to extension packages.
The Python Package Manager helps you installing additional packages that enhance Python. It determines the exact MacOS version and Python version you have and uses that information to download a database that has packages that are tested and tried on that combination. In other words: if something is in your Package Manager window but does not work you are free to blame the database maintainer.
PackageManager then checks which of the packages you have installed and which ones are not. This should also work when you have installed packages outside of PackageManager. You can select packages and install them, and PackageManager will work out the requirements and install these too.
Often PackageManager will list a package in two flavors: binary and source. Binary should always work, source will only work if you have installed the Apple Developer Tools. PackageManager will warn you about this, and also about other external dependencies.
PackageManager is available as a separate application and also as a function of the IDE, through the File->Package Manager menu entry.
See About this document... for information on suggesting changes.