SKOS is a lightweight language for representing intuitive, semi-formal conceptual structures. So, for example, the figure below (taken from the SKOS Core Guide) depicts concepts with intuitive hierarchical and associative relationships to other concepts, and with preferred and alternative labels in one (or more) languages — these are the kinds of structures that can be expressed using SKOS. Once expressed in this form, conceptual structures can easily be published on the Web, shared between applications, linked/mapped to other conceptual structures and so on. Typically, these conceptual structures are used as tools for navigating around complex or unfamiliar subject areas, for retrieving information across languages, and for bringing together related information from different sources.
RDFa is a language for embedding richly structured data and metadata within Web pages. This allows a Web page to expose much of its underlying meaning to applications, enabling a range of new functionalities within Web clients, exchanging data between Web sites, services, and the users’ desktop applications. For example, a Web page about a new music album can use RDFa to embed structured data expressing facts about that album, such as the track listing, artist, links to sample media files etc. A Web browser with a suitable plugin or extension can use this data to offer new functions to the user, such as download the tracklisting with available samples to my music library, or compare prices from online vendors.
Both of these technologies are on the W3C Recommendation track, and are scheduled for completion in April 2008.