dist-packages is a Debian-specific convention that is also present in its derivatives, like Ubuntu. Modules are installed to dist-packages when they come from the Debian package manager into this location:
/usr/lib/python2.7/dist-packages
Since easy_install and pip are installed from the package manager, they also use dist-packages, but they put packages here:
/usr/local/lib/python2.7/dist-packages
From the Debian Python Wiki:
dist-packages instead of site-packages. Third party Python software
installed from Debian packages goes into dist-packages, not
site-packages. This is to reduce conflict between the system Python,
and any from-source Python build you might install manually.
This means that if you manually compile and install Python interpreter from source, it uses the site-packages directory. This allows you to keep the two installations separate, especially since Debian and Ubuntu rely on the system version of Python for many system utilities.