Echelon is designed to intercept communication in form of fax, telex, e-mail messages, and phone calls. It is maintained by five nations: The U.S., Britain, Canada, Australia and New Zealand. Together they control a network of computers and interception facilities. The interception stations are placed all over the world so that most communication can be eavesdropped.
Intercepted messages are processed by computer systems that use filters to find relevant material. These filters consist of keyword lists that are maintained by the intelligence agencies of the involved countries. One of those keyword lists exists for each category of interest. Once a message is filtered and assigned to a specific category it is passed to the according central database.
Those persons with the permission to view the stored data are able to see the messages saved in a category. Several rules regulate which agencies are allowed to get to know what kind of information. Generally, all intercepted messages for a specific category are passed to that agency that created this category.
Such a system can hardly be used to keep a particular person under constant surveillance. But it is useful for special areas of interest, for example to get to know all communication about an organization whose name appears in messages about it. Because the keywords and related filter rules can be as complex as necessary it is possible to get quite accurate results.
But it has also to be said that on the one hand, Echelon has the potential to intercept millions of communications and can be used to violate privacy, but on the other hands, sorting and reading all the data is a real problem. There are simply not enough analysts to sort through everything.