The Petersburg-class transport was a type of Federation transport which entered service in 2351. Designed to transport Starfleet Ground Forces, these ships usually traveled in groups of several vessels and were escorted by a more combat-capable class. (RPG netbook: Eternal Vigilance: The United Federation of Planets Sourcebook, Vol. II)
The design of the Petersburg class began life as an early Jim Martin concept for the USS Defiant, which envisioned the design as a halfway point between a fully fleged starship and a Runabout.