I provided their charter. It seems like many people want to falsely over simplify the NSA mission into something different than what the document states.
In summary:
* NSA derives authority as a component agency under the DOD, a defense agency
* The primary mission is to collect signals intelligence and conduct information assurance
* perform as executive agency of operational security over inter-agency communications
* liaison with foreign governments to conduct sharing of classified materials for signals intelligence
There isn’t anything in the charter that specifically separates the NSA responsibilities or areas of operation from such of the FBI. The largest difference is that FBI is a law enforcement agency, as derived from the Department of Justice while the NSA is not. That means the NSA does not have fourth amendment interests which limits information sharing to the FBI. The Wikipedia page on the NSA documents a few scandals in this regard.