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, currently only for text 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.