Dave Berry (ESB Product Manager @Oracle) recently brought my attention to a discussion on the Oracle SOA Suite forum: Difference between Oracle ESB and BPEL. In short, the differences are in the problems they address. Confusion arises because there are similar capabilities in both. However, BPEL is about orchestrating business logic and ESB is about highly efficient integration.
The forum discussion is worth a read as there are useful contributions on which tool is right for what job, i.e. when to use BPEL and when to use ESB.