K-On! (or K-On!! for the second season of the anime) is a series of yonkoma by Kakifly about a group of high schoolers that decide to start a band. It was adapted into an anime by KyoAni in 2009, with a second season that ran through spring and summer of 2010. It is easily one of the most polarizing series of the last couple of years: it is difficult to discuss the series without causing a flamewar between its fans and detractors. It has a reputation as being the epitome of the moe style.
Most fans tend to either adore K-On! as cute and amusing or deride it as inane, stupid, and annoying. In particular, your opinion of the anime will depend greatly on how butthurt you were about Endless Eight (which was done by the same team at about the same time). Mentioning it in the same thread at /a/ as Haruhi is pretty much a guarantee of a flamewar.