The APT package currently contains two sections, the APT dselect
method and the apt-get command line user interface. Both provide
a way to install and remove packages as well as download new packages from
the Internet.
The Debian packaging system has a large amount of information associated with each package to help assure that it integrates cleanly and easily into the system. The most prominent of features is the dependency system.
The dependency system allows individual programs to make use of shared elements in the system such as libraries. It simplifies placing infrequently used portions of a program in separate packages to reduce the number of things the average user is required to install. Also, it allows a choices in for such things as mail transport agents, X servers and so on.
The first step to understanding the dependency system is to grasp the concept of a simple dependency. The meaning of a simple dependency is that a package requires another package to be installed at the same time to work properly.
For instance, mail-crypt is an emacs extension that aids in encrypting email with PGP. Without PGP installed mail-crypt is useless, so mail-crypt has a simple dependency on PGP. Also, because it is an emacs extension it has a simple dependency on emacs, without emacs it is completely useless.
The other important dependency to understand is a conflicting dependency. It means that a package, when installed with another package, will not work and may possibly be extremely harmful to the system. As an example consider a mail transport agent such as sendmail, exim or qmail. It is not possible to have two mail transport agents installed because both need to listen to the network to receive mail. Attempting to install two will seriously damage the system so all mail transport agents have a conflicting dependency with all other mail transport agents.
As an added complication there is the possibility for a package to pretend to be another package. Consider that exim and sendmail for many intents are identical, they both deliver mail and understand a common interface. Hence, the package system has a way for them to declare that they are both mail-transport-agents. So, exim and sendmail both declare that they provide a mail-transport-agent and other packages that need a mail transport agent depend on mail-transport-agent. This can add a great deal of confusion when trying to manually fix packages.
At any given time a single dependency may be met by packages that are already installed or it may not be. APT attempts to help resolve dependency issues by providing a number of automatic algorithms that help in selecting packages for installation.