The Helicarrier is S.H.I.E.L.D.'s command ship, carrying many agents, and Nick Fury. It also had a miniature prison (ironically called "The Big House") with many prison cells containing many of the world's dangerous superhuman criminals, until Gravitron broke them all out.
But it didn't survive the Skrull Invasion and is destroyed by Skrull infiltrators.
The Helicarrier possess the most advanced and powerful weapons systems and tech. It maintains the S.H.I.E.L.D aircraft, weaponry, equipment and combat suits.