Visualize a tetrahedron (a pyramid with a triangular base). Label each corner: "Metal", "Techno", "Singer/Songwriter", and "Rap". It is my theory that almost every "pop" or "rock" musical artist or group can fit somewhere in this tetrahedron. For instance:

Most artists aren't single points, but a sort of blob; their sound ranges around, depending on songs/albums. Some are rather limited (like Britney Spears), whereas some are pretty expansive (like Moby).

Within this theory, individuals are either "conservative" or "liberal" and "point", "edge" or "face" people. Me, I am a liberal-edge person, with the edge being "Techno" + "Singer/Songwriter": I love the extremes of these two genres, as well as stuff pretty close to the extremes in "Metal" and "Rap" -- but I just don't like getting down with Snoop Doggy Dogg or Pantera. My brother is a conservative-face person; he likes the extremes of Rap, Metal and Singer/Songwriter, but just doesn't like Techno at all. My parents are conservative-point around "Singer/Songwriter," with the "Metal" shared side being a little more liberal than the others; it's very difficult to get them to listen to our music.

Get the general idea? Okay, yeah, it's not a perfect system. Some groups are hard to place (like the Punk/Third-wave ska/Swing guys), some don't fit in period (the system makes no accomodations for orchestra/classical, jazz, or various ethnic influences, but most people can't visualize more than 3 dimensions without getting a severe headache), and sometimes two very different groups (like early REM and Simon and Garfunkel) will be right near each other on the scale. But for those groups/artists who do fit into the tetrahedron, it works remarkably well, especially as a rule of thumb for determining people's music likes and dislikes. If your head stops spinning long enough for you to start visualizing all of this.