Slice-of-life manga following a group of girls growing up together through high school with some cameos from teachers and a random cat. It's a bit like Garfield or other newspaper comics in that a joke is made in 4 panels and there's a thin overarching storyline to hold everything together. It's rare these days to find ink and paper writing or art which makes you really laugh out loud but that is what Azumanga gets you to do again and again and again.
It must be at least 5 years since I've read it and I still remember the characters and some of the randomer/crazier gags.
Besides the humour, the author/illustrator also does a good job of building relationships between the characters throughout the 4 volumes. If high school is already in the distant (or recent) past for you, this will make you miss all the great times you spent with friends and nostalgic for the freedom of youth.