(CNN) -- Ever since online music service Spotify launched in the United States last month, it has caused about as much confusion as it has excitement.
Partly that's because, at least on first glance, it doesn't seem entirely new. Spotify looks like iTunes -- only with greener colors. It wants Pandora-level popularity.
And it's built on the assumption that people want to rent music instead of buying it, much like Rdio, Napster (the legal version), Rhapsody and a host of others.
So what exactly makes Spotify different? And how do other services compare?
Here's a quick look at the current online music lineup:
It's a streaming music service, meaning you play tunes from the Web instead of your personal hard drive or phone. Premium subscribers can download songs, but Spotify has gotten so much buzz in part because it offers a robust service free.
Its catalogue of music is among the largest, with 15 million songs. Essentially, Spotify wants you to rent songs rather than own them. And, unlike on some other sites, you can choose exactly which tracks you want to listen to on this service -- instead of picking a genre.
With 100 million users, Pandora is one of the most popular online music sites in the United States. Visitors to Pandora.com choose a song title or an artist they like, and then Pandora's computers create a radio station out of songs that are musically similar.
So, if you create a station for Arcade Fire, you might hear some obscure indie artists mixed in with Coldplay and U2. Pandora is free up to a point. If you listen to music on the site for more than 40 hours per month, you have to pay $1 for the rest of the month. Yearly subscribers also get to skip the ads that are mixed in with Pandora's songs.
This one's big with the Silicon Valley crowd at the moment. Turntable.fm tries to emulate the concert-going experience -- you pick a venue and then listen to music of a certain genre, which is selected in real-time by real people. Users also can become deejays and select songs of their own.
So this free site is kind of a cross between Pandora and Spotify. You do get to choose some songs, like on Spotify. But you also spend lots of time listening to music that's chosen by other people who are online at the time -- like Pandora, except on Pandora, those people are online robots.
Apple's music player. It looks a lot like Spotify (or rather, Spotify looks a lot like iTunes), but there's an important difference: iTunes plays music you purchase and download; Spotify and others play music that you rent from the Internet.
(Apple also will launch a Web-based service, iCloud, this fall that will allow users to store and play music.)
Like Spotify, Grooveshark is a music streaming service, and one that gets good reviews from tech writers because it's easy to use and doesn't require an account for you to get started. Users upload their music into the company's library, and then listen to what they want for free -- if they don't mind hearing ads as well -- or pay a monthly fee for ad-free listening or to use the company's mobile app.
Google Music -- and, for that matter, Amazon's cloud services, too -- is designed to let you hear music that you've purchased on any computer, phone or Internet-connected gizmo.
The service, which is only open to those who have been invited, lets users store 20,000 songs in the cloud -- meaning on Google's servers.