Hopefully by now you've heard of the "Music Genome Project" known as Pandora, an experiment initially started by Billboard Magazine to take people's musical preferences and refine them down into a science for the purpose of suggesting new tracks or artists. The site has easily become one of the most popular internet radio web apps out there, and now Grooveshark has come along to try and improve on the winning formula.
One important thing to remember when using Grooveshark is that it is NOT Pandora, but aims to provide the same (or better) type of service in terms of streaming free legal music online. You do not need to create an account with Grooveshark, much like Pandora, although if you do, your preferences will be saved and loaded each time you login, which definitely enhances the accuracy of suggestions, as well as personalizes the whole experience.
The main differences between Grooveshark and Pandora are generally pretty small, but not altogether insignificant. Grooveshark allows you to customize the look of your player using various themes. It also brings the default search results to a number of songs by the artist that you initially search for. On Pandora, you're pretty much stuck with the look of the default player, and the stations you hear will generally only play a few songs by the artist you put in, mixed with a lot of other lesser known or related tracks.
As a huge music fans, I see each of these sites as being good for particular things. While Grooveshark allows instant satisfaction when trying to listen to a particular artist or track, sometimes I will prefer the simplicity of Pandora, as well as it's uncanny ability to suggest new tracks that not only have I never heard before, but match my tastes to a tee. However, I'm sure you'll get some great use out of Grooveshark and all it has to offer. Make your own comparison and dub your own victor in the comments!