It's staffed with Article III federal judges who are appointed by the Chief Justice, which makes its construction about as far from the executive branch as you can get.
It oversees the NSA and CIA, which are controlled by the executive branch, and has no authority to hear cases. It hears only matters regarding foreign intelligence, which puts its subjects outside the protection of the Constitution, which protections are most of the point of Article III courts. It is in that sense so difference from Article III courts as to make it a little unclear whether it is one.
And it was created by Congress, as a way of delegating rulemaking about surveillance that was properly owed by Congress --- the FISA process was a way for the 1970s Congress to punt on the issue. Unlike other courts, Congress can abolish FISA tomorrow and replace it with a set of rules. It is in that sense entirely a creature of Congress.
The FISC is weird, is what I'm saying.