When you buy an MP3 music file from a service like Amazon and install it in your MP3 player, you player already knows lots of information about the song, the album, the artist, and so on. This information, properly called "metadata", is stored in a special area at the end of the MP3 file so that players can use it. MP3 metadata is called "ID3 tags".
What is frustrating, however, is that while most hardware MP3 players show you the metadata while they are playing a song and when you are sorting your music, many don't. Here are a few of the MP3 players on my Mac: notice that none of them show the metadata such as artist and album name, and only the last one even shows the song title:
Firefox:
Safari:
Chrome:
VLC media player:
Note that the first three of these players actually can't know the metadata until they have downloaded the whole file; however, even after they have done that, they don't add the metadata to the display. To see this, open an MP3 that has ID3 tags, and notice that the MP3 player is unchanged.
The MP3 format is over 15 years old, and yet many software players are still rudimentary. I don't want to have to launch a separate program like iTunes or QuickTime just to see the title of the song I am hearing: my browser knows it, it just isn't telling me.