The short answer
Atta is whole wheat flour that keeps the bran and germ: it is browner, more filling and used for roti and paratha. Maida is refined white flour with the bran removed: smoother and softer, used for naan, samosa pastry and baking.
Which to use
| Make | Use |
|---|---|
| Roti, chapati, paratha | Atta |
| Naan, kulcha | Maida (or a mix) |
| Samosa, pakora batter | Maida / besan |
| Puri | Atta, or atta with a little maida |