Federation class

The Federation-class vessels were a type of dreadnought-type starships used by the Federation and Starfleet which operated during the latter half of the 23rd century.

Based on the standard Starfleet design tree and hull components of the time, the Federation-class starships were bulkier sisters of the heavy cruisers. The dreadnoughts employed a different sensor/navigational deflector / tractor beam dish arrangement (including an aft sensor dish) and most prominently, a third nacelle, all design elements largely abandoned in future starship development. 

Background
The Federation-class starship was originally seen on a display in the Starfleet Academy's training simulation in Star Trek II: The Wrath of Khan. The display was literally a page pulled directly from Franz Joseph's Star Fleet Technical Manual.

The Technical Manual also lists the USS Federation as being the prototype of the class.

The Motion Picture originally contained a voice-over in the beginning of the film for the dreadnought USS Entente, NCC-2120. This was a direct reference to the Technical Manual and its entries for Federation-class. (The manual was heavily used as a reference for the first film.) While this line of dialog remains intact on the laser-disk, VHS, and Beta releases of the film, the Director's Cut DVD explicitly muddles the track, making the line difficult to hear.

The concept of a three-nacelle starship has been derided by some as contrary to Gene Roddenberry's design requirements for Starfleet ships, violating both the "pairs" rule and the "nacelle line of sight" rule, but there is nothing in canon that mandates these rules. Indeed, while only the and the "off-timeline" version of the  are canon three-nacelled ships, there are many ships that violate the line of sight rules as well as several single nacelle vessels, including the, , and.

Though Gene Roddenberry personally signed off on the Federation class in 1973, by the time of Star Trek: The Next Generation, he was adamantly opposed to the "militarization" of Starfleet, specifically citing the Federation-class as problematic and therefore non-canon. This has caused some controversy and confusion in the years since, since the ship does appear as a schematic in canon, and one of her class is explicitly mentioned in the only Star Trek movie of which Gene Roddenberry himself was in charge.

Apocrypha
The Federation-class was included in the Star Fleet Battles series because the company was allowed to use material from the original series and the Technical Manual only.

The novel Dreadnought! by Diane Carey also featured a vessel of similar design, the USS Star Empire.

The game Star Trek: Tactical Assault also featured a variant of this class as a "Federation Dreadnought". According to the original source, a total of twenty vessels were planned: