Folk
Folk music began as music made by and for ordinary people, passed along by ear rather than by score, built around acoustic instrumentation and a directness between performer and listener that survives even as the genre moved from front porches to concert stages.
Traditional
The oldest function music has ever served — carrying a community's history, work, worship, and identity forward in time through song. This covers far more than dance tunes: work songs and sea shanties that kept labor in rhythm, marches and fife-and-drum music that moved people together, ceremonial dance forms like the Virginia reel or Irish jig. These weren't concert pieces, they were instructions for how a community worked, moved, and celebrated together. The composer is almost never known because the composer was almost never the point.
Leadbelly · The Carter Family · Burl Ives
Folk Revival
The moment a generation looked back and decided that what was being lost was worth saving. The Folk Revival of the 1950s and 1960s took the traditional forms and brought them forward with conscious purpose. These artists knew they were preserving something, and that awareness gave the music a moral weight that pure entertainment rarely carries.
Woody Guthrie · Pete Seeger · Joan Baez
Modern
Folk narrative architecture where the ensemble arrangement is structural rather than decorative. The band is doing compositional work; the song is conceived as an ensemble performance rather than as a solo composition dressed. Bob Dylan, Jason Isbell, Steve Earle, and Lucinda Williams classify here. Distinguished from Folk: Singer-Songwriter by whether the arrangement is load-bearing, not by how much arrangement is present.
Bob Dylan · Nanci Griffith
Singer-Songwriter
The song is built at a single instrument and the voice; arrangement, however rich, is color rather than structure. Strip the accompaniment and the song stands. Carole King, Nick Drake, James Taylor, Janis Ian, Jimmy Webb, Cat Stevens, and Don McLean classify here. The scope of the writing — personal or social — does not determine this classification.
John Prine · Townes Van Zandt · Joni Mitchell · Gordon Lightfoot · Kris Kristofferson