Data.gov.uk describes itself as an initiative that "brings [public data] together in one searchable website. Making this data easily available means it will be easier for people to make decisions and suggestions about government policies based on detailed information [...] There are over 9,000 datasets available, from all central government departments and a number of other public sector bodies and local authorities".
It was initiated by government and encourages people to participate through an online platform. Data.gov.uk is national in orientation and concerned with research, knowledge production & information access.
It was launched in January 2010 and is ongoing.