This site isn't new, but it is very powerful. Everything from jazz and R&B to Latin and classical is here. There are blogs to follow as well as a music gift guide.
Searching is easy - just type what you're looking for into the box at the top of the screen. Depending on your search you may see a long list of results with the most relevant at the top. Click the one you want, and you'll then get all kinds of information including a biography of the band/artist, discography, and awards. Entries are generally of decent length, like several paragraphs, and there are links everywhere so it's easy to find related information. You might even be able to listen to a snippet of a song.
The advanced search isn't really more complex; it just walks you through your search. Depending on your choices, you'll get more options until the "Go" button appears, which is what triggers the search. One cool thing you can do though is search text mentions. You won't get liner notes, but if your search term shows up in an allmusic.com review or article, you'll get to see that.
So now that you've found a CD, how do you know if you can purchase it or not? This may seem obvious, by try Amazon.com. You can try searching with allmusic.com's catalog number, but album title and/or artist may be the best option. Amazon also is pretty comprehensive in finding albums and song titles, and often you can hear about 30 seconds of a song. If an mp3 file is okay for you, you can possibly purchase one and have the music on your computer almost instantly. Otherwise, you'll have to order the CD. If it's not in print anymore, Amazon may have suggestions of online sellers.