Great manga for a laugh in between. It's basically about funny situations or conversations in a girls dorm, a dog and a bit of romance. It's episodic without too much background or overall plot. Although there are frequent "adult" jokes,there isn't much fan-service.
If you read Petit-tan you can recognize at first glance that it is from the author of Seitokai Yakuindomo. It has the same art, the same humor, and even the main cast consists of 3 girls (later 4) and one guy in both cases.
I actually prefer Petit-tan over SY (like them both though). There are mainly two reasons for that: One is that it isn't written in 4-koma style, which allows for more freedom with the placements of gags (but less frequent), the drawings seem better and the story feels a bit more cohesive. Additionally the characters feel more balanced in Petit-tan because they don't have to make a sexual joke every four panels.
The second reason is that the only guy in Petit-tan doesn't have the role of the tsukkomi. Since Tsuda always has to play the straight man in SY he sometimes seems a bit bland compared to all the other characters.