Wikipedia: Musical Genre
Brief explanation of the way styles can be defined by region, chronology, technical requirements, marketing trends, or the ideas of critics. Extensively linked to sub-genres and examples of significant artists.
http://en.wikipedia.org/wiki/Musical_genre

All Music Guide
Portal built around articles by music journalists describing music genres, with links and reviews.
http://allmusic.com/

Audiogalaxy: Glossary of Musical Styles
Hundreds of sub-genres defined in non-technical language mentioning sample artists. Organized under headings such as pop, hip-hop, jazz, blues, classical, Latin, modern rock, rock, and heavy metal.
http://www.audiogalaxy.com/list/glossary.php

Ectophiles Guide by Genre
Self-described "Guide to Good Music" from a group founded in 1991 to support the music of singer Happy Rhodes. Links to artists, almost entirely female vocalists, sorted by genre such as pop, blues, experimental, performance art, beautiful and fierce, and traditional.
http://ectoguide.org/genre

Music Web Hunter: Styles & Genres
Ken Davies has grouped resources in five categories: classical, folk/ethnic/world, jazz, musicals/operas/theater, and pop/rock/country.
http://www.kendavies.net/musicwebhunter/stylegenre.html

A Taxonomy of Musical Genres
Two French experts re-define genres to improve musical databases for electronic distribution, paper submitted to a conference in Paris in 2000.
http://www.csl.sony.fr/downloads/papers/2000/pachet-riao2000.pdf

Indiana University School of Music: Genres
Categorized links organized by researchers at the William and Gayle Cook Music Library. Categories include 20th-Century music, ancient music, band music, tango, flamenco, ragtime, and choral music.
http://library.music.indiana.edu/music_resources/genres.html

Music Classification by Genre: System Performance
Research project completed in 2003 by Mitali Banerjee at Rice University used automatic process to determine musical genre of audio samples.
http://cnx.org/content/m11690/latest/

America.gov: American Popular Music
Online publication contains stories that illustrate how Americans, borrowing from diverse musical traditions, have contributed to humanity’s universal language.
http://www.america.gov/publications/books/american-popular-music.html