"payload": [
  {  
    "id": String, // You can assume this be be unique
    "name": String, // The user defined name of the playlist
    "tracks": [
      {
        "id": String, // Unique ID for this song
        "index": Number, // The index position (starting at 1) of the track in the playlist
        "title": String, 
        "artist": String, 
        "album": String, 
        "albumArt": String, // URL to the albumArt for this song
        "duration": Number, // Duration of song in milliseconds
        "playCount": Number // Number of times the user has played this song
      }
    ]  
  }
]

This data is sent every time the user adds, deletes or modifies their playlists. With some users this will be a lot of data so be conservative with how you handle it as it can cause lag when people have lots of songs in their playlists.

The payload will contain 0 to many playlists and each playlist will contain 0 to many songs.