What are the topics I should learn for a BPM (Business Process Management) product? I mean, do I need to learn BPMN (Business Process Modelling Notation), BPEL (Business Process Excecution Language)? Are there any open source lightweight BPM products by Apache or somebody?
If you are implementing a BPM product, you should know a business process modelling spec like BPMN and an execution language like BPEL and BPML.
Otherwise, if you're looking for application development using a BPM Product, you don't need to know those specs.
Some open source BPM suites to explore and use,