Scorpio Corvettes are some of the smallest, most under-powered ships in the Federation Starfleet. Built in the late 2270's to supplement the firepower of patrol cruisers and other system monitors, the Scorpio Corvettes were built around a pair of low power point defense phasers and a light photon torpedo. Unfortunately, Starfleet engineers could not resolve the problem of dangerous feedback loops in the plasma coils. This lead to the design only being able to operate either the phasers or the photon torpedo - never both - without shutting down either sensors or shielding, neither of which were amiable solutions to the problem.
The closest thing that the Federation came to actual fighter craft, the Scorpio Corvettes were eventually sold off or scrapped once it became obvious that a single Excelsior Cruiser could handle the missions that required an entire flotilla of Scorpios.
The Scorpio design did have lasting effects for Starfleet designers. Though the idea of flotillas of small craft were put to rest by the project, the Scorpio Corvettes did lead to the concept of small, independent ships that can be used for short-range duties. Because of this, the Scorpio can be thought of as one of the original 'runabout' designs.