Katon is one of the five basic elemental nature transformations. You can use fire release by molding chakra inside of your stomach, then releasing it through your lungs and mouth. Most Katon techniques are mid to long range, and deal combust ive/explosive damage.
Katon users seem to mainly stem from Konohagakure. The Sarutobi and Uchiha clans are both families that use fire release, and the Uchiha clan is said to have a natural affinity for Katon. Also, the Uchiha are the first to show the ability Amaterasu, which is unlocked via Mangekyō Sharingan, and Amaterasu is said to be the most powerful of fire techniques.
Katon is weak to Suiton, because naturally, water extinguishes flames. It is also superior to Fūton, because wind only fuels a fire, as shown by Naruto Uzumaki and Sasuke Uchiha's combination attack. It is speculated that Katon relates most to Raiton, since both wreak heavy destruction.
Known elemental combinations are fire+water(boil release), Fire+earth(Lava Release), fire+wind(scorch release), and fire+wind+earth(dust release).
Anything else I need to expand upon, Senpai?