One of the big stinks in Trekdom is trying to figure out a sane, rational nomenclature for ship types. I have never really favored the traditional naval nomenclature for starships, but I figure they hung on for a while, at least until specific ship-roles became apparent. My thesis is that these were just getting shaken out during TOS, refined in the Movie era, and were pretty well defined sometime prior to the 2340's. I think role works better than type, as it allows for some rationale in ship size dimorphism.

TOS/Movie Era
Small, fast ships with long duration, designed to rip through systems, noting and plotting features, events, anomalies, planets, ETC. These ships to not tarry, nor seek contact. Loaded with sensors and electronic equipment. Few shuttles.Scout (Hermes)Scout (Voyager)
Medium or large long duration ships designed to tarry for great periods in one area, studying things in detail. Lots of space, lots of shuttles and lots of sensors. Large science crews and many labs. Capable of handling multiple assignments simultaneously.Frigate (Miranda/Avenger)Surveyor (Nebula)
Medium or large long duration ships designed to visit new areas, tarrying and studying for a while, making sure object under study is safe or warrants follow-up with detailed study. Lots of sensors and weapons, some science labs and a moderate complement of shuttles. Heavy Cruiser (Constitution, Excelsior)Explorer (Galaxy, Ambassador)
Medium or large short duration ships designed to patrol and protect a given system, and to serve as front-line combat ships. Few sensors, few shuttles, small crews. May be assigned to a sector or starbase for very long periods. May spend much time "In Reserve".Cruiser/Destroyer (Constellation/Gengis)Warship (cruiser/destroyer, etc) (Steamrunner)
Medium or large short duration ships with no particular specialty, designed to fill in gaps where needed. May serve as tenders to other ships. Light Cruiser General Purpose (GP) (Akira)
Small, medium or large ships designed to haul large amounts of material. Not fragile merchant ships, but full-fledged starships optimized for cargo rather than sensors, weapons or shuttles.
Small, medium or large ships designed for (or customizable to) one specific science-related project or role. GrissomOlympic

So.. in the TNG era, it goes like this: A Scout rips through a new system, counting and classifying planets, noting those inhabited, and any other odd stuff, and makes a report to Starfleet, ripping out again probably the same day. Some time later, an Explorer shows up and starts poking around, assessing threats and civilizations, making contact if warranted. The Explorer fleshes out the Scout's report, spending a couple days doing so. Perhaps the system needs further study, so some time after the Explorer leaves, a surveyor shows up, perhaps with a diplomatic team or some form of advanced ground party. The surveyor (or its team) may spend months in the system, perhaps supported by a network of General Purpose ships and smaller vessels, possibly including medical or science ships. Once and a while, a sector warship passes through on routine patrol.