ScummVM is a virtual machine for running classic DOS games. Originally, ScummVM was intent for playing only SCUMM games. SCUMM stands for Script Creation Utility for Manic Mansion – an LucasArts engine for creating games. Most notable games created in this engine are Manic Mansion, Monkey Island series, Day of the Tentacle, Full Throttle, Sam & Max. Later ScummVM added support for lot of non-SCUMM games by implementing other game engines such ADL, AGI, SCI, ADRIFT, AdvSys, Level9, ZCode etc. This now gives it ability to play over 200 games.
ResidualVM is sister project of ScummVM and is capable of running GrimE (Grim Engine) games: Grim Fandango and Escape from Monkey Island. On a October 10 2020. it is announced merging of ResidualVM into ScummVM.
ScummVM is free open source project but many of games are still under license so you need original datafiles. ScummVM only replaces the original game executables allowing you to play them on new systems.