A task force was a grouping of personnel or starships that were brought together to undertake a specific mission.
In early 2368, Captain Jean-Luc Picard of the USS Enterprise-D successfully persuaded Admiral Shanthi of Starbase 234 to launch a blockade force to the Klingon-Romulan border, to prevent Romulan forces covertly supplying the House of Duras by setting up a tachyon detection grid. Picard's task force was formed by commandeering all ships in the base's shipyards, along with all ships within one day's travel of the starbase. (TNG: "Redemption II")
In 2369, Vice Admiral Nechayev assembled a 15 ship fleet consisting of five task forces with the stated aim of defending Federation colonies from an anticipated second Borg invasion. Admiral Nechayev would command the fleet from the flagship USS Gorkon. One of these task forces, designated task force three, was commanded by Captain Jean-Luc Picard and consisted of the USS Agamemnon, the USS Crazy Horse, and the USS Enterprise-D. (TNG: "Descent")
In 2372, a Klingon task force, led by the new flagship IKS Negh'Var. arrived at Deep Space 9. The task force was supposedly there to assist the Federation in defense of the Alpha and Beta Quadrants, but in reality the station was being used as a staging post for an invasion of the Cardassian Union. (DS9: "The Way of the Warrior")
Several days later, Admiral Hastur commanded a task force of six ships led by the USS Venture to Deep Space 9 after it was attacked by the Klingons. Other starships in the task force included two Excelsior-class ships and a Miranda-class starship. (DS9: "The Way of the Warrior")
Admiral Gilhouly led another task force to Deep Space 9 in 2373, to help the station fight against a possible Dominion attack. The task force was later bolstered with the addition of Klingon and Romulan starships. The task force was almost destroyed after a Founder attempted to detonate a bomb in the Bajoran sun using the USS Yukon. Fortunately, he was stopped by the USS Defiant. (DS9: "By Inferno's Light")
In 2379, a Starfleet task force, Star Fleet Battle Group Omega, was deployed in Sector 1045 to counter an impending Reman incursion. Several weeks later, Captain William T. Riker of the USS Titan led a task force to the Romulan Neutral Zone to engage in direct talks for a final peace between the two sides. (Star Trek Nemesis)