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 and 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. In this context, using the mOSAIC Cloud ontology and Semantic Engine, cloud applications' developers will be able to specify their services and resources requirements independently from Cloud providers' specific solutions. In order to update and maintain the platform and the mOSAIC API, the mOSAIC Semantic Discovery Service will, on the other hand, discover Cloud providers' functionalities and resources, and will compare and align to the mOSAIC API, thus supporting agnostic and interoperable access to Cloud providers' offers.