A package manager is software that allows a user to install new software, upgrade the system, or update specific software, among other tasks. On Linux-based systems, where software often has many dependencies that must be present on the system for complete installation, tools like package managers become essential on every system.
Each Linux distribution ships with its default package manager for the above-mentioned functionalities, but the most commonly found ones are: Yum on RHEL and Fedora systems (where it is being replaced by DNF from Fedora 22+ onwards) and Apt on Debian-based systems.