Echelon is reportedly run by the USA primarily, in conjunction with the UK, Canada, Australia and New Zealand. I'm not sure how much information was ever declassified, but the system was initially created as a method to monitor Soviet communications during the Cold War.
Apparently it's essentially a giant keyword based system that sifts through text and perhaps even voice searching for combinations of words - such as Allah, bomb, President, nuclear etc in the same text. This stuff is then sent to analysts (obviously highly filtered due to the volume of traffic). I think it's more about identifying specific patterns of message rather than reading the individual ones, however.
Australia, New Zealand, and the Netherlands have however confirmed that Echelon does exist, although not the specifities of its operations. Former CIA Director R. James Woolsey admitted using the system to uncover information about foreign companies using bribes to win contracts, as a specific example.