Shuffle play randomizes the track or album order. Tracks are always played in the order displayed. Every time you press either shuffle button a new random sequence of tracks is generated and displayed. If tracks are currently playing the new order may change the next track played. The next track played is always the track just after the last played track in the displayed list.
The album shuffle button randomizes the order of albums, but preserves the track order within an album. Two tracks are considered to be in the same album if their album fields are identical and they are in the same directory. This algorithm is not perfect, but it works with the most common software to digitize and download albums. Generally the tracks from an album are stored in the same unique directory named for the album.