Cloud vendor lock-in and interoperability gaps arise (among many reasons) when semantics of resources and services, and of Application Programming Interfaces is not shared. Standards and techniques borrowed from SOA and Semantic Web Services areas might help in gaining shared, machine readable description of Cloud offerings (resources, Services at Platform and Application level, and their API groundings), thus allowing automatic discovery, matchmaking, and thus supporting selection, brokering, interoperability end even composition of Cloud Services among multiple Clouds. The EU funded mOSAIC project (http://www.mosaic-cloud.eu) aims at designing and developing an innovative open-source API and platform that enables applications to be Cloud providers' neutral and to negotiate Cloud services as requested by their users. Using the mOSAIC Cloud ontology and Semantic Engine, cloud applications' developers will be able to specify their services and resources requirements and communicate them to the mOSAIC Platform and Cloud Agency. The mOSAIC Cloud Agency will implement a multi-agent brokering mechanism that will search for Cloud services matching the applications’ request, and possibly compose the requested service.