The X/Open DTP Model is a widely-supported standard that defines components and interfaces useful for dealing with distributed transactions.
In this model, the application uses a transaction manager to help carry out a global transaction. The app communicates with the transaction manager via the TX interface, which provides the transaction semantics (begin/commit/rollback/etc).
Each data store involved in a global transaction is exposed to the transaction manager as a resource manager. The communication takes place over the XA interface, which defines functions for lower-level transaction semantics. The transaction manager uses the XA interface to carry out two-phase commits, and the resource manager uses it to dynamically enlist specific resources in the transaction.
Nuts and Bolts of Transaction Processing, by Subbu Allamaraju, subbu.org
Global Transactions - X/Open XA - Resource Managers, by Donald A. Marsh, Jr., Aurora Information Systems, Inc.