Tempora is a GCHQ programme that stores all data flowing through UK fibre optic cables so that it can be analysed by GCHQ staff. Tempora is the parent project of two other programmes Mastering the Internet and Global Telecoms Exploitation.
According to the Guardian, Tempora "allowed the agency to set up internet buffers so it could not simply watch the data live but also store it – for three days in the case of content and 30 days for metadata."
"The processing centres apply a series of sophisticated computer programmes in order to filter the material through what is known as MVR – massive volume reduction. The first filter immediately rejects high-volume, low-value traffic, such as peer-to-peer downloads, which reduces the volume by about 30%. Others pull out packets of information relating to "selectors" – search terms including subjects, phone numbers and email addresses of interest. Some 40,000 of these were chosen by GCHQ and 31,000 by the NSA. Most of the information extracted is "content", such as recordings of phone calls or the substance of email messages. The rest is metadata."
The Guardian states that
The document identified for the first time which telecoms companies are working with GCHQ's "special source" team. It gives top secret codenames for each firm, with BT ("Remedy"), Verizon Business ("Dacron"), and Vodafone Cable ("Gerontic"). The other firms include Global Crossing ("Pinnage"), Level 3 ("Little"), Viatel ("Vitreous") and Interoute ("Streetcar"). The companies refused to comment on any specifics relating to Tempora, but several noted they were obliged to comply with UK and EU law.