It was an invite-only torrent tracker. What set it apart was its size. Any song you could think of, in any format you could think of. You could find lossless music that there's 0 chance that you could find anywhere else (legally or not).
Artists launched their own albums there first and became famous, the so called "Vanity house". An example of an artist that got big is "The Flashbulb".
What sibling comments said, also, great community, great recommendations, relationship graphs between artists/styles, very neat and tidy metadata (accurate tags, releases with serial numbers, etc).
It was approaching all music in all formats, ever.
Formats yes, but more importantly for music fans and audiophiles, all mediums!