K-ON! (The Manga/Anime) and K-ON (aka Keiongaku or Light Music in Japan)

K-On! (けいおん! Keion!^?) is a Japanese four-panel comic strip seinen manga written and illustrated by Kakifly. 

A 13-episode anime adaptation produced by Kyoto Animation aired in Japan between April and June 2009.

K-On!'s story revolves around four Japanese high school girls who join Sakuragaoka Girl's High School's light music club (Keion-bu) to try to save it from being disbanded, however, they are the only members of the club. 

The title of the series comes from the Japanese word for light music, keiongaku (軽音楽^?), which in the Japanese context is similar to pop music.