There are currently 148 champions in League of Legends as of January 2020. League divides its champion types up a number of ways. The most salient difference is the type of damage a champion deals; some champions deal largely physical damage, which is resisted by the armor stat, and other champions deal largely magic damage, which is resisted by the magic resistance stat.
Some champions deal with a combination of both and can choose which to emphasize; some rare abilities deal 'true' damage which is not mitigable by either armor or magic resistance. Riot Games has classified all champions as one of six types to aid beginners. Not all champions perfectly fit their type, of course.