The Kazon are a violent warrior species native to the Delta Quadrant. They had at least two racial variants, one with brown skin and one with more common copper-colored skin. The foreheads of all Kazon featured distinctive ridges and their black or brown hair grows in large chunks rather than individual strands. Kazon society is divided into a number of sects that fight each other over territory and resources. As of 2372, there were 18 different sects in the Kazon Collective.
Until 2346, the Kazon were a subject race to the Trabe. The Trabe treated their Kazon subjects brutally, and kept them in line by constantly turning the sects against each other and encouraging in-fighting. However, Jal Sankur convinced the various sects to join together and revolt against their Trabe oppressors. They were successful and rid the homeworld of the Trabe, while obtaining many Trabe vessels and a lot of technology. (VOY episode: "Alliances")
Using the Trabe ships and technology they began to spread out across a sizable portion of the Delta Quadrant. However, the Kazon sects soon turned towards fighting each other again, so technological development was no longer a primary concern. While they had developed plasma weapons and deflector shields, they had not developed transporter or replicator technology. (VOY episodes: "State of Flux", "Maneuvers")
In 2371, the Caretaker pulled the Federation starship USS Voyager 70,000 light years across the galaxy. The Kazon - specifically the Ogla sect - was one of the first species the Voyager crew encountered in the Delta Quadrant. (VOY episode & novelization: Caretaker)
For the next two years the crew of the Voyager had to contend with the threat of the Kazon - who wanted to capture the ship, which was filled with advanced Starfleet technology. In 2373 the Kazon-Nistrim succeeded in briefly capturing Voyager, but Tom Paris, the Doctor, and Lon Suder were able to retake the ship. (VOY episodes: "Basics, Part I", "Basics, Part II")