"Frere Jacques" is a traditional French nursery rhyme, and this is the Round version of the Song since it is frequently sung in this way. One child or group will start singing the melody, and then a second child or group will join in 2 bars later, and so on. If there are sufficent children or groups then four separate parts can continue to sing the round, each unit starting again at the beginning of the song after they have reached the end. The standard (non-Round) version of the song is available at this link: Frere Jacques.
You can play the mp3 file and see the sheet music if you scroll down the page. The PDF sheet music, Midi and Mp3 files can be downloaded using the links in the left-hand menu. This version of Frere Jacques is in the key of F and it has 2 parts. A fun version of Frere Jacques in the key of C with 5 parts can be seen in this video: