GUID (or UUID) is an acronym for 'Globally Unique Identifier' (or 'Universally Unique Identifier'). It is a 128-bit integer number used to identify resources.
Guids are Globally Unique Identifiers, also known as UUID's, Universally Unique Identifiers.
They are 128-bit pseudorandom values. There are so many valid
Guids (about 10^18
Guids for each cell of every people on Earth) that if they are generated by a good pseudorandom algorithm, they can be considered unique in the whole universe by all practical means.
Guids are most often used as primary keys in databases. Their advantage is that you don't have to call the database to get a new ID that is (almost) guaranteed to be unique.