The best place to learn about XDI is the XDI Graph Model.
XDI.ORG is an international non-profit public trust organization governing open public XRI and XDI infrastructure. XRI (Extensible Resource Identifier) and XDI (XRI Data Interchange) are open standards for digital identity addressing and trusted data sharing developed at OASIS, the leading XML e-business standards body. XRI and XDI infrastructure enables individuals and organizations to establish persistent, privacy-protected Internet identities and form long-term, trusted peer-to-peer data sharing relationships.
XDI Charter on IC Site