A formation of four Constitution-class starships, 2268
A ship class was a shared construction design of spacecraft or starship operated by an entity.
Vast research and testing efforts went into perfecting a new ship class before construction, such as the Galaxy Class Starship Development Project by Starfleet or the development process of the D7 class by the Klingon Empire. (TNG: "Booby Trap", DIS: "Point of Light")
Starfleet's primary hub of ship class research and development was Utopia Planitia Fleet Yards on Mars. (TNG: "Booby Trap", DS9: "The Search, Part I")
In 2365, Geordi La Forge implemented a change to the design of the dilithium crystal orientation aboard the USS Enterprise-D. Upon viewing the modification, one of the ship's designers, Leah Brahms, observed that his modification was due to be introduced in the next class starship. (TNG: "Galaxy's Child")
Starfleet often continued the Earth tradition of naming the first starship of a class after the class (such as the USS Excelsior, the first Excelsior-class starship launched) or the USS Prometheus (the first Prometheus-class starship). These vessels often bore the registry prefix NX. Starfleet also commonly continued the tradition of listing a starship's class on its dedication plaque. (Star Trek III: The Search for Spock; VOY: "Message in a Bottle")
The term sister ship was often used to describe vessels of shared lineage. (ENT: "The Expanse", et al.) By 2267, at least twelve Constitution-class ships had been commissioned by Starfleet. (TOS: "Tomorrow is Yesterday")
Some Klingon ship classes used a "D" prefix before a number (i.e. D4 class, D5 class, D7 class, or D12 class.) (ENT: "Judgment"; Star Trek Into Darkness)
In the late 24th century, a Galaxy-class starship was often the point of comparison for ship classes. The Duras sisters admitted that their Bird-of-Prey was "no match" for a Galaxy-class starship. In 2371, Admiral Patterson described the Intrepid-class as "not as big as a Galaxy-class, but she's quick and smart..." (Star Trek Generations, VOY: "Relativity").
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Shared ship classes have had the added benefit to Star Trek of reducing production costs and production time when a new ship is needed, especially in the age of studio models.
The Star Trek: The Next Generation Technical Manual portrays a twenty-year design and construction process of Galaxy-class ships, although "Booby Trap" and "Eye of the Beholder" seem to suggest that this timeline was much faster. The Defiant-class, launched in 2371, was said to be developed in response to the Borg, which first presented themselves as a threat in 2365/2366.