The Business Process Modeling Notation (BPMN) is a graphical notation that depicts the steps in a business process. BPMN depicts the end to end flow of a business process. The notation has been specifically designed to coordinate the sequence of processes and the messages that flow between different process participants in a related set of activities.

BPMN is targeted at a high level for business users and at a lower level for process implementers. The business users should be able to easily read and understand a BPMN business process diagram. The process implementer should be able to adorn a business process diagram with further detail in order to represent the process in a physical implementation.
BPMN is targeted at users, vendors and service providers that need to communicate business processes in a standard manner.

What is the relationship between BPMN and BPEL (Business Process Execution Language)?

BPEL is an XML-based language for describing a business process in which most of the tasks represent interactions between the process and external Web services. The BPEL process itself is represented as a Web service, and is realized by a BPEL engine which executes the process description. BPMN is a standard set of diagramming conventions for describing business processes. It is designed to visualize a rich set of process flow semantics within a process and the communication between independent processes. It is intended to support capture of sufficient detail to allow it to be the source of an executable process description. Since BPEL is currently considered the most important standard for execution languages, a translation to BPEL is specified in the BPMN standard. By design there are some limitations on the process topologies that can be described in BPEL, so it is possible to represent processes in BPMN that cannot be mapped to BPEL. There are a few concepts, such as Ad-Hoc sub-processes, that BPMN can represent that may not be implemented with any technology.

Reference:

http://www.bpmn.org