Talos IV was the inhabited M-class fourth planet of the Talosian system, which was located in the Talos star group. The planet was the homeworld of the Talosians, a telepathic humanoid species, as well as of the Talosian singing plant. The planet had an atmosphere of oxygen-nitrogen and a gravity that was 0.9 of Earth standard. (TOS: "The Cage", "The Menagerie, Part I")
In 2254, the position and orbital path of Talos IV in an inner system map was displayed on the bridge of the USS Enterprise. This planet, located three astronomical units from the binary stars Talos A and Talos B, had a highly inclined orbit. (TOS: "The Cage", "The Menagerie, Part I", production art)
After thousands of years had passed since war had devastated the planet, Talos IV was beginning to support life again in the mid-23rd century. By then, the planet's deep underground had been inhabited by the Talosians, who had evolved as an old and weakened species with enormous mental powers. In 2254, the planet was visited by the Federation starship USS Enterprise – under the command of Captain Christopher Pike – after receiving a distress call faked by the Talosians supposedly from the SS Columbia, that had crash-landed there in 2236. However, the only survivor of this crash was Vina, a female Human. (TOS: "The Cage", "The Menagerie, Part I")
In 2267, Talos IV was revisited by the Enterprise while commandeered by Spock, in order to deliver Captain Pike there. Though Pike had been crippled by delta-particle radiation poisoning, he was able to live a normal life on the planet, together with Vina. (TOS: "The Menagerie, Part II")
In 2328, this was the destination for passengers traveling from Triskelion Orbital Station via commercial transport. Ryan Adams, Wynsdey Adams, Loren Bivens, and Svetlana Reeves-Stevens traveled on the SS Kogin, and Mike Echols traveled on the SS Wisconsin. (TNG-R: "Inheritance", okudagram)
Background information Edit
In the original story outline for "The Cage" (as fully reprinted in the reference book The Making of Star Trek, pp. 47-65), Talos IV was named Sirius IV and was inhabited by crab-like aliens who kidnapped Captain Robert April. The outline additionally described the surface of Sirius IV as "arid, rocky" and "the arid waste."
Near the end of August 1964, Gene Roddenberry wrote a memo to his production manager, Jim Paisley, which asked such questions as, "Regarding the surface of planet Talos IV, does the present description of the planet seem practical as regards available locations? Is the description and action presently in the rough draft suited for matching set construction, i.e. stage shooting of the more complicated dialogue portions of these Talos IV planet surface scenes." (Star Trek Creator: The Authorized Biography of Gene Roddenberry, p. 205)
In the script of "The Cage", Talos IV was described as "a violet-orange planet."
The set used for Talos IV's surface was built on Stage 16 of Desilu's Culver City lot.  The set was designed by art directors Franz Bachelin and Pato Guzman. It included a massive painted backdrop that greatly enlarged the set's apparent size. Although the sky pictured in the backdrop was a cloudy gray, Director of Photography William E. Snyder and the production crew gave the sky a slight greenish tint using colored lights, thereby making the planetscape seem more alien. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD) The Talosian elevator created as part of the set was built on a platform, allowing the lift to descend without necessitating excavation below the soundstage's floor. ("The Menagerie, Part I" text commentary, TOS Season 1 DVD) Underground interiors of Talos IV were filmed on Desilu Culver Stage 15. 
By 14 September 1964, a budget of US$6,400 had been allocated for creation of the special effects necessary to show the exterior of Talos IV in "The Cage". (Star Trek Creator: The Authorized Biography of Gene Roddenberry, p. 206) To portray the planet as seen from the Enterprise upon approach in the original version of "The Menagerie, Part II", footage of Talos IV was reused from "The Cage". The film clip was upgraded and matted into the Enterprise's bridge viewscreen. ("The Menagerie, Part II" text commentary, TOS Season 1 DVD) The 2006 remastered versions of "The Menagerie, Part I" and "The Menagerie, Part II", as well as the 2008 remastered version of "The Cage", feature several updated, CG shots of Talos IV.
According to Star Trek Maps (chart D), Talos IV had two moons called Mone and Toh.
According to the Star Trek: Star Charts (p. 34), Talos IV was a quarantined and non-aligned planet. Information on the planet's official name, government, capital city and population were not available to the Federation in the late 24th century. The Talosians were warp-capable 500,000 years ago.
According to The Worlds of the Federation, any distress signal or other communication supposedly originating from anywhere in the entire Talosian region was to be assumed false, with no exceptions. According to the reference book, the indigenous name of the planet was Clesik.(citation needed • edit)