Carnatic music is not a folk tradition (although it does have some folk influences like most music). It is a highly classical form and it shares the same framework with Hindustani music. Hindustani (Ravi Shankar etc.) is North Indian classical music, whereas Carnatic is South Indian classical music, mainly practiced in the four southern states of India.
1. Both systems are melody based i.e.raga based. And both are highly evolved in their own way in that aspect.
2. With carnatic music, the rhythmic framework (tala) is also quite elaborate and highly sophisticated and that is a big difference with Hindustani.
3. The # of basic diatonic scales in practice in Carnatic music is quite a lot (72). So you get to see truly exotic combinations
4. In Carnatic, a significant % of notes (swaras) carry pitch ornamentations, grace notes etc. These are called “gamakas” in Indian classical music. Hindustani also has some, but carnatic music melodies are full of them. In general, gamakas are a defining characteristic of carnatic music.
5. I think Carnatic music has stricter rules. It is also considered as more representative of the older form of Indian classical music with Hindustani having a very strong persian, moghul influence. But both systems have borrowed concepts from each other as well.

I have a post in my blog about Ragas and how they differ from Scales.

Arun