Python APT’s library provides access to almost every functionality supported by the underlying apt-pkg and apt-inst libraries. This means that it is possible to rewrite frontend programs like apt-cdrom in Python, and this is relatively easy, as can be seen in e.g. Writing your own apt-cdrom.
When going through the library, the first two modules are apt_pkg and apt_inst. These modules are more or less straight bindings to the apt-pkg and apt-inst libraries and the base for the rest of python-apt.
Going forward, the apt package appears. This package is using apt_pkg and apt_inst to provide easy to use ways to manipulate the cache, fetch packages, or install new packages. It also provides useful progress classes, for text and GTK+ interfaces. The last package is aptsources. The aptsources package provides classes and functions to read files like /etc/apt/sources.list and to modify them.