The Web Services for Remote Portlets specification defines a web service interface for accessing and interacting with interactive presentation-oriented web services. It has been produced through the efforts of the Web Services for Remote Portlets (WSRP) OASIS Technical Committee. It is based on the requirements gathered and on the concrete proposals made to the committee.
Scenarios that motivate the WSRP functionality include:
Content hosts, such as portal servers, providing Portlets as presentation-oriented web services that can be used by aggregation engines.
Aggregating frameworks, including portal servers, consuming presentation-oriented web services offered by content providers and integrating them into the framework.
More information on WSRP can be found on the official website for WSRP. It is suggested that you read the primer for a good, and albeit technical overview of WSRP.