In its basic form, folk music is traditional songs passed on by performance more so than recordings (mainly because recording didn't exist back then), usually on acoustic instruments indigenous to particular places and peoples, and springing from a common heritage of balladry and traveling minstrels, usually on subjects topical to the times, whether that was love or other more material matters. Ethnic divisions within folk music were distinct and varied, with each geographic area developing instruments, sounds and style unique to those places, many of which melded in later years as people traveled greater distances and were exposed to many varied influences.
Folk music as we have come to know it commercially is based on these old forms and styles, but with subject matter branching frequently into the political or socio-economic realm. It reached its height of popularity during the Great Folk Scare of the 1950s and 1960s, only to come to a crashing end when Dylan went electric.
Seriously though, folk music has continued to live on in modern times through reinterpretation by artists all over the globe, incorporating folk motifs and instrumentation in more modern settings. I like to think it has a common appeal for most of us, as it tugs at those "mystic chords of memory" Lincoln spoke about, and that we share in our ancestral DNA. It's music from, and about, our roots.
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