BPM - Business Process Management, some people also use it for Business Process Modeling.
BPMN - Business Process Modeling Notation - A standard notation orgininally defined by BPMI and subsequently owned and released by OMG. Pl note it is just a notation and has no serialization. It does define a mapping to BPEL4WS.
BPEL - Business Process Execution Language - This is the OASIS standard for orchestrating web services to execute business processes.
BPMS - Business Process Management Suites - A term used for product (or a set thereof) that provides definition and execution of business processes.
BPDM - Business Process Definition Metamodel - A meta model proposal for serializing BPMN diagrams. It is in RFP stage.
XPDL - XML Process Definition Language