-It is nowadays commonly considered a required for PackageManagement to also address dependencies - which means it should make sure to at least notify the SystemAdministrator when they're trying to install a [Package] that won't work because it depends on other software not currently installed. For example, some [GUI] application may require [GTK+] and/or [GNOME] libraries to run. These are of course also provided as [Package]s. Those may again depend on other packages, which may in turn have their own dependecies - and so on. 

-As you can imagine, installing any not entirely trivial package can take ages and turn out to be a very frustrating job. Therefor, most contemporary [PackageManagementTool]s were designed to ease the laborous task of meeting all the dependencies of a particular package by resolving them automatically. Binary based distributions usually rely on ''front-ends'' such as [APT] built on top of their original more simple-minded [PackageManagementTool]s, while source based distributions tend to have intelligent dependency management by default. To be able to automatically resolve dependencies, such [PackageManagementTool]s use an external repository (or several ones) of [Package]s which .