The folks over at ProPublica have compiled an
extraordinary  package of stories and
graphs, from past and present, that explain the NSA’s surveillance program as
well as other wiretapping efforts used by law enforcement agencies in the U.S.
They have also created a timeline of U.S. spying techniques dating  back to the 1978 Church hearings, which led
to the creation of the FISA court.

You can read the material here, here and here.