Japanese Manga written by Kiyohiko Azuma, and published monthly in Dengeki Daioh Magazine, one of the large phonebook-size manga magazines popular in Japan.
It deals with the exploits of a group of middle shool girls in typical situations that may arise in japanese schools. The cast of characters is very entertaining and allows for a wide variety of stories. There is the sporty one, the dumb one, the young genius one (everybody just loves cute Chiyo-chan), the kowaii (scary) one who nonetheless is the one who loves cute things the best, the chaotic teacher, the "I-love-young-girls" scary teacher and many many more. Just read it and enjoy the fun. AFAIK, there are scanlations on the web somewhere.
A typical strip has a standard length of 4 vertically arranged panels with a punchline in the last panel, although some storylines also saw regular manga format stories.
Thus far (June 2002), 4 volumes have been published and it seems that the latest volume also ends the series, as the girls graduate from middle school and in the latest Dengeki no new stories were to be found...
There is however a TV-show retells the manga in its episodic nature, that is quite nicely done. It is being broadcast in TV Tokyo after midnight every Thuesday. The was also a short Azumanga Daioh movie shown in conjunction with the new Slayers movie, the DiGi Charat movie and the Sakura Taisen movie.