(covers information from several alternate timelines)
An assault team, or strike team, was a group of commandos, soldiers or security officers that was sent along on away missions in case of hostility, or beamed aboard starships or outposts. Assault teams were typically more heavily armed with weapons and specialized equipment for combat situations including rifles, thrown and melee weapons.
During the 22nd century, United Earth used groups of MACO soldiers aboard Starfleet vessels for assault teams, most notably aboard the NX-class of ships. An assault team was used in April, 2152 to steal data disks containing ship's logs that proved Enterprise innocent of destroying the Paraagan II colony. (ENT: "Shockwave")
In June of 2151, Lieutenant Malcolm Reed was in command of an assault team of two which beamed down to the Vulcan monastery at P'Jem to end the hostage situation of the Andorian Imperial Guard. (ENT: "The Andorian Incident")
In 2153, Corporal F. Hawkins was standing by with an assault team after Jonathan Archer discovered where the Xindi-Arboreals were processing kemocite, but Archer kept him on standby, stating that he wasn't ready to send in the troops yet. (ENT: "The Shipment")
In the mirror universe, the Terran Empire's Starfleet employed assault teams in a wider variety of situations, and usually superseded the away team instead of complementing them. In 2155, Commander Jonathan Archer led an assault team that boarded the USS Defiant. Archer again led an assault team to find the Gorn Slar and recover the starship components the alien had stolen. Archer also used an assault team to arrest all the aliens aboard the ISS Avenger. (ENT: "In a Mirror, Darkly", "In a Mirror, Darkly, Part II")