The Qo'noS system was a star system located in the Beta Quadrant. Qo'noS, the capital planet of the Klingon Empire and homeworld of the Klingons, was located in the system. (Star Trek Into Darkness display graphic )
According to the display graphic from Star Trek Into Darkness the Qo'noS system was located in the Omega Leonis sector block. Omega Leonis is a real star 112 light years away from Earth. The Qo'noS system was presumably located closer to Earth than that, as the episode ENT: "Two Days and Two Nights" marked the first time a starship from Earth had traveled as far as 90 light years from the Sol system, and the Enterprise NX-01 had already visited Qo'noS in the pilot episode of the series.
The primary of the system was first seen in TNG: "Redemption" as the USS Enterprise-D and IKS Bortas approached Qo'noS. According to the Starfleet Medical Reference Manual, the Klingon homeworld was the planet "Epsilon Sagittarii B". Epsilon Sagittarii is also a real star located 143 light years away from Earth. According to the Star Trek: Star Charts (page 56), the star was an orange dwarf. According to Star Trek Maps and The Worlds of the Federation name the star was "Klingon". According to Star Trek Maps (Charts A & C), the system had an orange binary star and five planets. The Klingon homeworld was the second planet in the system.