Dynamic Range refers to the amount of difference in volume between the quiet parts and the loud parts of a piece of music. Loudness refers to the overall perceived volume of a piece of music relative to another. The human ear is easily fooled into believing that 'louder' sounds 'better'. In fact, music with more dynamic range sounds better because it is more natural. It is this perceived difference in loudness which is exploited by the commercial music industry; artists and labels have been competing with one another to make their tracks sound louder than the one before, to have a more attention-grabbing impact on the listener. This state of affairs is known as 'The Loudness Wars'.
Music is made louder by compressing (or squashing) the dynamic range, resulting in the loss of its natural peaks and troughs. This allows the the overall volume to be turned up, because the peak loudness of the song is limited. Or put another way, the quieter parts of the song can become louder without the louder parts becoming louder; the more compression, the louder the track will seem. But unfortunately squashing the dynamic range in this way has some undesirable musical outcomes: distortion, less definition and separation of instruments, compromised sense of space and depth, less power and impact, unnaturally constricted dynamic range, and listener fatigue. Fidelity is sacrificed.
The good news is that things are changing. There is a concerted push by those in the music industry who prefer fidelity and dynamics in music to end the 'Loudness War'. A universal standard for measuring loudness, EBU R128, has been widely embraced, and some countries have introduced laws to implement that standard. This means that audio playback levels will be determined by average loudness rather than peak loudness: in a nutshell, dynamic music will now sound louder and more open, while overly compressed material will sound quieter and squashed (the way things should be). iTunes Radio has adopted this approach, and currently it's optional in the iTunes program: just turn on 'Sound Check' in the playback preferences. Another great outcome of the adoption of this standard is that TV ads won't be louder than movies – a screaming ad for furniture won't be louder than the Death Star exploding, good eh?
In the meantime while these industry shifts are beginning to take place, you may notice that the Strange Days On Earth album is quieter than other music in your library. This is to be expected and the solution is simple: just turn it up!