Building Information Modelling (BIM) is about modelling and improving information flow throughout the construction lifecycle. Originally, the term applied to building construction projects, but it now encompasses infrastructure through to operations and maintenance.
OpenBIM is about recognising the need for vendor-neutral (non-proprietary) methods of exchanging information throughout a project. The OpenBIM concept is explained in this video by our colleagues in buildingSMART Norway:
What is openBIM
What is IFC (Industry Foundation Classes)
Working with IFC and Model View Definitions
The buildingSMART Data Dictionary
The BIM Collaboration Format (BCF)