SAP Global Data Types (GDTs) are SAP-wide unified data types representing business-related content built according to the UN/CEFACT CCTS. All elements of SOA services are described (typed) by GDTs. They are the basis for the harmonization of the SAP SOA services.
This WIKI gives a brief introduction in what Global Data Types are and how SAP is designing Global Data Types.
Furthermore a complete catalog of all Global Data Types defined by SAP is contained in this WIKI.