There are many definitions but broadly speaking they can be defined as the senior HR strategists and change agents aligned to the senior leaders within the business that create the people strategy in support of the broad business objectives.
In theory they are freed of the administrative burden (allocated to shared services) and are not expected to be expert in all areas (devolved to centres of expertise) but they are required to have the expert, inside knowledge of their particular company, division or business unit,
They are the ones that should understand where their business is, where it has been and more importantly where it is going so that effective people initiatives are designed and implemented.
This requires many new skills such vendor management with internal and external sources of supply. Agreeing to and keeping to service level agreements. Working in a more collaborative regime rather than through direct line control.
Many traditional HR managers may not make the transition and even those that can will need, development, support and coaching interventions. There is no “one size fits all” model but there will be an approach that suits your organisation.