GUID: Globally Unique Identifier is 128-bit value that is a unique reference number. It is 32 hexadecimal digits with groups separated by hyphens. Upon generation you can choose to strip the hyphens and if all chars must be uppercase.
Machine Key: Machine Keys are used by ASP.NET for encryption, decryption and validation of forms-authentication, view-state data and other purposes. This is perticually used in webfarms where all of the server nodes must have the same machine key in order to encrypt/decrypt/validate request/responses across multiple nodes. The generated section must be placed/replaced in the section in the configuration file.
A random number generator is used, so the output is different each time you generate a new.