A package manager in Linux is a tool that facilitates the installation, removal, upgrading, configuration, and management of software packages on an operating system. It can be either a graphical application, such as a software center, or a command-line tool like apt-get or pacman.

In simpler terms, a package refers to an application, be it a graphical or command-line tool, or a software library essential for other programs. These packages are archive files containing the executable, configuration files, and sometimes details about dependencies.

In the realm of Linux, package managers play a crucial role in simplifying the installation, removal, and management of software packages. The choice of package manager often depends on the underlying packaging system employed by the Linux distribution.

RPM Package Manager (RPM): RPM is a packaging system primarily used by Red Hat-based distributions such as Fedora, CentOS, and RHEL. Two popular command-line package managers associated with RPM are Yum (Yellowdog Updater, Modified) and DNF (Dandified Yum). Yum was the predecessor to DNF and is commonly found in older RPM-based systems, while DNF is the modern and preferred package manager for handling RPM packages on more recent distributions.

DEB Package Manager: DEB is the packaging system used by Debian-based distributions like Ubuntu and Debian itself. Command-line package managers for DEB packages include apt-get and aptitude. These tools streamline the process of fetching, installing, and managing software on Debian-based systems.

Coexistence of Package Managers: It's worth noting that within a given packaging system, multiple package managers may coexist. For example, on a system using the RPM packaging system, both Yum and DNF can be present. However, it's common for newer distributions to transition from Yum to DNF, and in such cases, DNF becomes the preferred package manager.

Having multiple package managers within the same packaging system allows users to choose the tool that aligns with their preferences or system requirements. It also ensures a smooth transition for users accustomed to a particular package manager as newer tools are introduced.