Basic infantry units who can take-and-hold fortifications or activate Forerunner artifacts.
UNSC Marine infantry are armed with assault rifles that are effective against infantry but deal little damage against ground vehicles. They can receive additional reinforcements and supplies in the field, including the ability to throw grenades or assigning a combat mechanic to the squad. The mechanic will automatically repair nearby damaged vehicles and is armed with an anti-tank rocket launcher.
Marines are the UNSC's expeditionary force-in-readiness, and after decades in cryo-sleep they are old school both figuratively and literally. The Spirit of Fire's veteran Marines are experienced at fighting in exotic environments and have received training by Anders to operate Forerunner systems well enough to avoid triggering defense wards and immolating themselves [most of the time].