What the heck is a Raag?

This is how I understand it simply: A Raag is a set of rules for creating music/melody.
The rules of a Raag will determine things like:

• which notes to skip and which notes to use - overall
• which notes to skip and which notes to use when you're ascending from low to high
  
• which notes to skip and which notes to use when you're descending from high to low
• what pattern of notes must be included in the music you're creating
  
• what pattern of notes must be avoided in the music you're creating

Staying within the rules of each Raag - you can improvise as much as you wish.

That's basically it, without getting into too much detail.