The Advanced Package Tool (APT) provides a toolkit for Linux users who want better control over their software packages. Options for APT include apt-cache, apt-get, apt.conf, sources.list and apt-secure. It is most commonly used for installing new packages, including new options for Bash that aren't native to a particular flavor of Linux.
APT-Cache is used for querying APT's internal database. The database caches the information from sources listed in the sources.list file. The usual syntax is apt-cache search keyword, in which keyword is a word or phrase that you want to find in the database. When querying the APT-Cache, it helps to be as specific as possible with your search terms. Apt-cache search lib might give hundreds of entries, while apt-cache search therion gave me three entries for a software that's useful for 3D modeling and surveying of caves along with related packages.
APT-Get is most commonly used for installing new packages along with dependency trees, which makes it popular with Linux users who want to install software without hunting down that one dependency that's causing their entire installation to fail. In most cases, using apt-get requires Super User privileges, so the syntax may look something like, sudo apt-get update, which pulls down updates for your Linux system.
Using APT-Get for installations usually uses the syntax of sudo apt-get install xfs, in which xfs is a mechanism by which an X server can communicate with a font renderer. APT-Get will automatically build a dependency tree, calculate the amount of memory needed and should usually prompt you to continue with the installation once it has calculated this information. To remove packages, sudo apt-get remove xfs is a good option.
Sudo apt-get dist-upgrade is useful for upgrading distributions of your packages. It works much like apt-get update and will prompt you to continue once it has gathered the necessary information and also prompt for whether you want to keep your existing version (N) or replace it with the new version (Y).
APT-Secure provides archive authentication support for APT. APT contains code that provides signature coding for the release file of its archives to ensure that unauthorized modifications to packages with a Release file signing key are not made. APT-Secure is the last step in the chain of trust, meaning that it checks the archive maintainer but not packages themselves. A trusted package may still contain malicious code, so using tools like debsig-verify and debsign are recommended before using APT-Secure. Linux users should also take steps to prevent malicious activity such as “man in the middle” attacks and events that compromise the security of a mirror network.
APT.Conf is a configuration file for every APT tool and can be used to look up and modify options. When an APT tool is used, it will check the APT file so that it can operate. It is organized into a tree similar to directory trees, but with the syntax APT::tool::option. For instance, using APT-get install will call the install option within the get tool within the APT group of tools.
Sources.list is a package resource list used to locate archives for the packaging system used by the Debian GNU/Linux system. Sources.list can support many active sources and types of source media. The list contains information such as the source type, the Universal Resource Identifier (URI) that is a superset of the Universal Resource Locator (URL) and the ARGS information for each source entry. It is located at /etc/apt/sources.list.
Is there anything else I can do with APT?
There is, but I recommend studying each tool in the APT toolbox before using them because, whenever you work with Terminal and Bash, you're working with some powerful tools that can really cost you if used incorrectly. Typing in man apt will give you a bit more information (Not much – that man file is pretty sparse for man but typing in, say, man apt-get will tell you everything you want to know about apt-get.) but will give you an idea of which tools are available with Advanced Package Tool.