From Halopedia, the Halo wiki
A fleet is a large, permanent organization of ships including starships. Fleets are usually relatively large and manned by thousands of individuals on what is normally dozens or hundreds of ships. Smaller, more temporary groups of ships are generally known as battle groups.
In 1573, Japan had a fleet of 133 ships which were destroyed by 13 ships. At a different time point, the United States of America also had a Pacific fleet at Pearl Harbor, which was eventually destroyed by Japan.
Defensive fleets are typically named after the star system they are based in. Prior to the Human-Covenant War, they were considered little more than glorified revenue police, though they proved invaluable in providing training cadres and battle-ready (if outdated) warships to the war effort. Defensive fleets are typically built around smaller "tactical" carriers serving as the fleet's core. Known defense fleets include;
Expeditionary fleets are numbered fleets designed for offensive operations into deep space. These fleets were largely decimated in the early years of the Human-Covenant War, and by the war's end there was no appreciable difference between Expeditionary or Defensive fleets, and the navy was unable to conduct any kind of offensive operation without leaving Earth and the remaining colonies dangerously vulnerable. During the period in which they were operational, Expeditionary Fleets built their cores around massive super carriers such as the Epoch-class heavy carrier. Known expeditionary fleets include;
By the end of the war, many of the Navy's fleets had also become "paper fleets", or "dead fleets", with no command and no ships. These fleets were assigned to sectors lost to the Covenant early in the war, with their destroyed squadrons remaining attached to the sector to honor their sacrifice and as deference for tradition. A new ship was occasionally inadvertently attached to one of these dead fleets due to bureaucratic error, though such assignments were considered ill omens by their crew members.
UNSC fleets are comprised of numerous battle groups, smaller tactical formations of ships. Naval fleets rarely patrol together, and it is fairly common for single ships to patrol regions of space alone. Fleets are also comprised of squadrons, though by the end of the war these were administrative rather than tactical groupings. Fleets, as with battle groups, usually have at least one prowler to act as the fleet's eyes and ears and to also serve as saboteurs.
Covenant fleets are usually dominated by larger, more powerful ships. Covenant frigates are common in smaller fleets, however, there are usually far more destroyers and cruisers suited for plasma bombardment. Covenant fleets are essentially self-sustaining armadas, capable of operating without supply for indefinite periods thanks to their use of agricultural support ships. Like Covenant ships, fleets are also given different designations mostly pertaining to religious nomenclature. Larger Covenant fleets typically contain thousands of ships, most of them CCS-class battlecruisers and groups of CAS-class assault carriers.
Fleets were a basic unit of organization in the Forerunner ecumene's military forces. Throughout Forerunner history, most fleets fell under the authority of the Warrior-Servant rate, but after their reduction at the hands of the Builders, Builder Security became the dominant military organization. Forerunner fleets were under the authority of Fleet Command.