genesis,
Let me try and give a couple of examples. The first one from 'kaatha jaadhi'. Song sung by Mahathi, 'enna marandalum'.
http://www.thiraipaadal.com/albums/ALBIRR00285.html
Observe how Mahathi starts the song. Without any backing instruments, it is clear it is a 'village' song. The tune is very clearly 'folk' or 'village type' (if I can use such a phrase.) Once that is established, then world music takes over :) When the pallavi starts the accompaniment is congos and drums which are not really folk or village instruments. The first interlude again has guitar, violins with flute providing the counterpoints. The second interlude has the guitar again playing with the keyboards and the metal part of the drums. None of these again village instruments. Inspite of all these instruments getting into the song, what remains is the haunting emotion of the girl and her sorrow. An amazing number which only a genius can compose.
Or from the same movie check out 'katre katre' probably the best song Karthik has sung. The folk part is unmistakeable in the prelude flute and the pallavi of the song. In the first interlude check how the synthesizer cooperates with the flute (synth nowhere close to being a folk instrument) and see how the charanam has changed with the drums accompanying the tune now and the phrasing also changed a lot. Still the folk element of the tune is not changed!!! Not sure how he does that. The second interlude starts with tavil and the nadaswaram, carnatic instruments which play a folk tune and again the flute and synth come in. So many different instruments, change in phrasing but the soul is still folk.
When I wrote about this movie once, Plum said it was a 'Platinum Standard'. I vehemently agreed with him.
A more famous song would be 'madura marikozhundu' song. After the 'pacharisi maavu' is completed, there is a brief flute to which the keyboard provides the counterpoints. The first interlude starts in the WCM fashion with the violin and flute counterpoints. The whole song is based on the carnatic raga 'Mayamalavagowla'. Listen to Chitra's entry and the violin backing. Pure WCM stuff. The second interlude again is WCM if you look at how each instrument counters the other or it is carnatic if you see how 'Mayamalavagowla' is developed. So much happening in this song but overall it will be clear to everyone who listens that it is a folk song, everything else be damned :D