Do you like music? Of course you do, unless you're a 92 year old hermit. If do, GET OFF MAH LAWN BLOG!!!
Anyways, Spotify is great in many elements. Why? Two words: FREE MUSIC!
Ok not COMPLETELY free, but I'll talk about that later.
Spotify is really handy for me to use pretty much because I'm always on Firefox (a fox-addict has to use Firefox after all) or Minecraft, and Spotify never lags out your computer.
Another good reason why Spotify is so handy is because it has so many songs, all for free. If you like Skrillex, AWOLNATION, MGMT, Bon Jovi, Neill Diamond, Coldplay, Nirvana or whatever, you can listen to them. You can listen to them all in a playlist if you want (although Skrillex and Neill Diamond aren't really the best combination)
A very handy feature in Spotify is the ability to share music with friends, Facebook or otherwise. So you too can send classical music to your 16 year old cousin who will probably stuff you in the bin the next time he sees you.
There's also an ability to follow an artist, meaning that when that persons/bands new song/album comes out, you will be notified by a handy sidebar on the right.
One of my favorite parts about Spotify is the fact that there's a "Discover" tab. This tab collects data on what songs you've recently listened to, and compiles this together to search for songs or artists that you may like. And boy does it work well; I've gathered about 15 new favorite artists in the 3 months I've had it. I'd never even heard of Moby until Spotify came along.
Now onto the free/not free thing. I myself have bought Spotify Premium, which isn't necessary but VERY handy unless you want to listen to just 10 hours of music a month.
Well, It's 10 hours over in Ireland anyways.
Spotify Premium also has another feature: the ability to save songs so that you can listen to them offline. This way, if you have absolutely terrible internet connection that was make a snail proud, then you can listen to a song offline to stop all this lag nonsense.
All in all, Spotify is an extremely handy desktop application if you like music, and even if you don't like any of these modern artists the Discover tab will make sure that you find something good.