What is it that NSA does?

What are or were its legal parameters and its history? This is a quick "NSA 101" course that might be helpful as stories continue to emerge about the agency. Oddly, as large as this organization is, it has been very much in the background, and only recently when some whistleblowers spoke up, has this agency gained a good deal of public attention. Some of you may recall the fuss raised about some spy agency named Echelon and wonder how this group is or is not connected to NSA. And soon at least one whistle blower will testify before congress, though the White House seems to have convinced some 50% of Americans that the president can do whatever he wants in time of war, ignoring legal constraints upon intel branches. And that raises the question (for me): if NSA can skirt the courts to "fight terror," then what of the FBI, also once requied to have court approval for phone taps. Are they too now free to do as they want in this "fight against terror"? [from MetaFilter.com]