Peterr asked this question on the Clifftop thread and I thought it deserved a separate thread.

My 1st stab:

"Old time music" is a catchall term used for music based on the country music recorded 1922-1945, and on the old fiddlers who learned their repertoire from even older people about the same time. As played today, it's played in bands where the fiddle is almost always the lead instrument, the banjo is played in clawhammer (frailing) style. Sadly, the singing in most of those old 78s is no longer emphasized, but some of us are trying to Do Something about that and sing Carter Family, Monroe Brothers, Charlie Poole, Riley Puckett's repertoire.

"Bulegrass" grew out of old time music and can be dated back no earlier than 1945, when Bill Monroe first added Earl Scruggs and Lester Flatt to his band. It features LOTS of good singing in two and three part harmony, is often played in keys where there are no open strings on the fiddle and mandolin (B and B flat!) so that the lead singer can "show his stuff", and the instrumental breaks are traded among 3-finger Scruggs-style banjo, mandolin, fiddle, and sometimes dobro and lead guitar.

   Oh boy. . .rereading this I realize that every name or concept mentioned could be fractally expanded. . .