GS1-128, formerly knowns as UCC/EAN-128, is a global standard used by different companies to exchange data between themselves. Principally, GS1-128 encodes data and provides a method of defining the meaning of the data.
In simple term, GS1-128 is a standard that defines the type of data and what the data format should be. In itself, it is not a barcode, but the guidelines for formulating the barcode.