George Kirk was a Human Starfleet officer in the early 23rd century. His father was Tiberius Kirk. He was the husband of Winona Kirk and the father of James T. Kirk and George Samuel Kirk. (TOS: "Where No Man Has Gone Before", "Operation -- Annihilate!", Star Trek)
James Kirk often spoke of his father, crediting George with providing him with the inspiration to join Starfleet. George lived at least long enough to see his son become captain of the USS Enterprise in 2265. (Star Trek)
George Kirk was played by Chris Hemsworth.
Although Spock referred to George Kirk as being a lieutenant when he was aboard the Kelvin, Kirk actually wore the rank braids of a lieutenant commander; Spock may have been abbreviating his rank in a non-traditional manner (lieutenant commanders are usually afforded the privilege of being referred to, in short, as commanders and not as lieutenants).
George Kirk was mentioned in the unfilmed Star Trek: The First Adventure script as having died when his craft, the Bonaventure, disappeared during an experimental dilithium-fueled warp jump. Montgomery Scott worked with him on the project.
He may have a brother, since James T. Kirk said he was staying at his uncle's farm in Idaho in Star Trek Generations. It was never made clear which of his parents had a sibling though, or if he was just using the term for a long time family friend. In Star Trek, an uncle Frank was cut from the script and reworked into Winona's new husband heard in the film.
Intel's Star Trek tie-in website gave his serial number as SA-733-9624-AM.
George Kirk was seen as the first officer of the USS Enterprise under Captain Robert April in the novel Final Frontier. In the novel, Kirk was the one responsible for convincing Captain April (who had been charged with naming the as yet-unnamed ship) to name the ship Enterprise. The cover art for the book contained a picture of him - and it bears a clear resemblance to Hemsworth, even though the actor was only four years old when the book was published.