UUID (Universally Unique Identifier): A sequence of 128 bits that can guarantee uniqueness across space and time, defined by RFC 4122.
GUID (Globally Unique Identifier): Microsoft’s implementation of the UUID specification; often used interchangeably with UUID.
UDID (Unique Device Identifier): A sequence of 40 hexadecimal characters that uniquely identify an iOS device (the device’s Social Security Number, if you will). This value can be retrieved through iTunes, or found using UIDevice -uniqueIdentifier. Derived from hardware details like MAC address.