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

DataDragon: Music Genre Sampler
Children's site with simple definitions of a few genres including rock, Celtic, and classical with artist and site links.
http://datadragon.com/education/genres/

Christian Music Place: Artists by Genre
About two dozen musical genres including blues, a capella, dance/techno, and Southern gospel, with as many as several hundred links to Christian music artists in each.
http://www.christianmusic.org/cmp/artists/index.cgi?command=Display_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

Representing Musical Genre: A State of the Art
Journal of New Music Research article from 2003 discusses manual, prescriptive, and emergent approaches to genre classification for the European CUIDADO project, Content-Based Unified Interfaces and Descriptors for Audio/Music Databases Available Online.
http://www.idiap.ch/~paiement/references/to_read/music/genre_classification/pachet-02c.pdf

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

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/