From Halopedia, the Halo wiki
In the UNSC Navy, frigates form the backbone of any fleet or battle group, while cruisers appear to fill a similar role within the Covenant. As their most ubiquitous warship, the UNSC uses frigates in a broad range of roles, ranging from heavy air support to fighter escort and fleet engagements. Some UNSC frigates are equipped with a sizable cargo bay for larger dropships and armor complements. The UNSC has also employed light frigates equipped with stealth systems in a specialized reconnaissance role. Covenant frigates are not as commonly seen as their UNSC counterparts, but one of their roles appears to be escorting larger ships such as destroyers or carriers.
A number of frigates appear in several Halo games. Frigates are also units available to use in the table top game Halo: Fleet Battles.