Starfleet Tactical (or Starfleet Tactical Command) is a division of Starfleet that is responsible for the designing, planning, and implementation of both offensive weaponry and defensive measures aboard Starfleet vessels and facilities.
Starfleet Tactical faced their biggest challenge in 2365, following the USS Enterprise-D's encounter with the Borg in System J-25. It had been proven that current Starfleet weaponry were ineffective against the Borg, so many new weapons and starship designs were initiated by Starfleet Tactical including the Defiant-class and Prometheus-class and weapons such as the quantum torpedo. (TNG episodes: "The Best of Both Worlds, Part I", "The Best of Both Worlds, Part II"; DS9 episode: "The Search, Part I")
Following William T. Riker's "creative strategy" to rescue Captain Jean-Luc Picard from the Borg in 2367, Riker believed that the term that was used at Tactical was "what in blazes will Riker do next?". (TNG eBook: A Sea of Troubles)
Long-range Threat Assessment and Response Division
This branch was asked with monitoring the frontier for any signs of trouble. Jean-Luc Picard was assigned to this division of Starfleet Tactical after the Manraloth were freed from their stasis. (TNG novel: The Buried Age)