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. The term GUID is generally used by developers working with Microsoft technologies, while UUID is used everywhere else.
GUID has many usage when you are into programming. Just to create an Identity of Anything and you are not sure what sequence you can use and you want it MUST Be Unique then GUID is the best solution.
To Generate new GUID NameSpace required is only System which is always in the using area. You can get new GUID in C# just by calling Guid.NewGuid() without generating object of any class.
GUID is SQL Server is called as UniqueIdentifier. Most developers use UniqueIdentifier in SQL Server as Reference of the data generated at runtime. Using UniqueIdentifier as primary key not harmful but it is performance killer in SQL Server.
NEWID() Function is used in SQL Server. This commands helps you generate new GUID/UniqueIdentifier at runtime. You can also use this command to generate multiple ID in Bulk.