It works with iTunes. Its technically a standalone program but it accesses your iTunes songs and changes them accordingly.
I use it and love it, its the most accurate tagging system I've used (I've only used one other and that was Tune Up). Keep in mind though no tagging system is perfect so you have to watch what it tags.
Once it gets the metadata for each item in your library it shows the old one next to the new one and you pick which one you want to use, this keeps the program from overwriting custom data.