There's not really much that can be said about this album that hasn't been said anywhere else.  This is a classic, in every sense of the word.  It's the album that Pink Floyd is most known for, it staying on the Billboard 200 for over 14 years before finally being knocked off.  The cover art of the prism with a beam of light being converted to a rainbow is one of the most instantly recognizable images in music, right up there with Sgt. Peppers and Abbey Road.

Now how does the music stand up?  The psychedelic space rock sound that the band had been developing over the years on their albums is basically perfected here.  Conceptually, Dark Side is all about human insanity and paranoia; every track on here is dedicated to a facet of it.  Sound effects and various interviews with the mentally insane (including the now famous line "Here today, gone tomorrow!") are scattered throughout for cohesion.  Every track blends and cuts into the next, making this the type of album that you listen to in one sitting, rather than the usual album where you only listen to certain tracks.  Of course, Floyd was never about singles, and more about albums, but it's more pronounced here.  Dark Side also started the Floyd trend of using the ooooohs and aaaaaahs for backup vocals, that would continue into every album after this.

The remarkable thing about Dark Side of the Moon is that despite the fact that none of the songs on here became hit singles(that honor would go to The Wall's Another Brick In The Wall), that the album was as successful as it was.  And once hearing it, it's really not hard to see why.  The album as a whole coheres remarkably well, and I'd go as far to say that the only other album that they did that's better constructed is this is their next one, Wish You Were Here. 

On a personal note, though, if I had to choose what would be my favorite track(s) on this album, it'd definitely be Time, Money, and the album closers, Brain Damage and Eclipse, which have a beauty in them in their complex mix of keyboard and guitar that to my ears have not been reproduced anywhere else.  The "I'll see you on the dark side of the moon" line on BD and the ending hum never fail to send a shiver down my spine.

In short, if you for some reason haven't heard this record yet, download it, or better yet, buy yourself a copy.  It is an absolute classic, and no music fan should be without it.