From Halopedia, the Halo wiki
A fireteam is the smallest organization that ground forces are assigned to in the UNSC Army, Marine Corps and Spartan branch. As the smallest unit, it serves as the base upon which all other units are organized from. The closest organizational unit in the Covenant military is known as a lance.
The concept of the fireteam is based on the need for tactical flexibility in infantry operations. They generally consist of approximately four soldiers or Marines who are formed into a squad. Generally, a squad contains three fireteams.
A fireteam is capable of autonomous operations as part of a larger unit. Successful fireteam employment relies on quality small unit training for troops, experience of its members operating together, sufficient communications infrastructure, and a quality non-commissioned officer corps to provide tactical leadership for the team.
Note: The number of troops, their armament, and their roles in the team may vary from team to team. For example some teams may include an anti-matériel specialist, a demolitions expert, a designated marksman, a combat medic, or other mission-critical personnel. In many cases, this specialized function replaces the basic rifleman position in the fireteam. The following is just an example. This model is based off of modern military fireteam structure, and is (unless specifically noted) not fully canon to Halo.
Special Operations Forces
In the Spartan Operations branch, Spartans are typically divided into fireteams of 4-5 personnel. Spartan-IV fireteams are led by a Spartan with the rank of Fireteam Leader, roughly analogous to a UNSC Captain.