The Yellowdog Updater, Modified (YUM) is a free and open-source command-line package-management utility for computers running the Linux operating system using the RPM Package Manager. Though YUM has a command-line interface, several other tools provide graphical user interfaces to YUM functionality.
YUM allows for automatic updates and package and dependency management on RPM-based distributions. Like the Advanced Package Tool (APT) from Debian, YUM works with software repositories (collections of packages), which can be accessed locally or over a network connection.
Under the hood, YUM depends on RPM, which is a packaging standard for digital distribution of software, which automatically uses hashes and digisigs to verify the authorship and integrity of said software; unlike some app stores, which serve a similar function, neither YUM nor RPM provide built-in support for proprietary restrictions on copying of packages by end-users. YUM is implemented as libraries in the Python programming language, with a small set of programs that provide a command-line interface. GUI-based wrappers such as YUM Extender (yumex) also exist.
A rewrite of YUM named DNF replaced YUM as the default package manager in Fedora 22. DNF was created to improve on YUM in several ways – improved performance, better resolution of dependency conflicts, and easier integration with other software applications.
Sources: Wikipedia