Acronym for Globally Unique IDentifier
A system for generating identifiers so they will be globally unique (ie, no two people will ever generate the same ID). This can be used to uniquely identify some resource. These are heavily used in a lot of MicrosoftCorporation's Software, and have had reasonable adoption by other people too.
Some methods for generating GUIDs have gotten a (deservedly) bad rep, such as including the host computer's MAC address in the GUIDs generated by some of MicrosoftCorporation's software which allowed correlating GUIDs from the same machine to each other, potentially even tracing them to their origin. This includes various versions of WindowsMediaPlayer and MicrosoftOffice.