Good overview of BPEL: http://www.radikalfx.com/bpel/language.html
BPMN vs BPEL: http://www.infoq.com/articles/bpelbpm
Interestingly most Process Engines, including IBM’s Process Manager support direction execution of BPMN (exported as XPDL’s or directly drawn in the associated tooling)
Another standard at play is SCA which was developed jointly by many SOA integration vendors. It plays a complimentary role to BPEL.
SCA and BPEL – Rivals or Friends?
Sometimes, when talking about composite service-based applications, people get confused about the roles of SCA and of BPEL and consider that these two technologies are in conflict or that they are trying to perform the same roles. This isn’t the case – far from being rivals, SCA and BPEL are firm friends and are complementary parts of a business solution, each with its own role to play.
Putting this in another way, SCA is concerned with what components exist in the business application, what services those components offer, what service references those components depend on, how the components are connected together, what endpoint addresses and communication methods are used for the connections, what policies are applied to components and to the connections between them. BPEL is concerned with business logic and the sequences of operations which are performed to execute an individual business process. BPEL processes provide and consume other services through partnerLinks, but these are abstract interfaces that must be connected to actual endpoints and communication methods through configuration.