Parathas are buttered chapattis - you place knobs of butter on a circle of the uncooked dough, roll it all up into a swiss roll shape, squeeze gently, then form a spiral, press flat and roll out again to form a circle 1/8" thick (you may need to dip the dough in flour if it gets sticky), then place on the hot tava (or just spread some melted butter over the first circle of dough it and then cook it).
Roti's are (to me) stuffed chapatti's - roll out two little circles of dough separately, spread over one of them some dryish cooked spicy lentils or spicy mashed potato, plonk the other circle of dough on top, pinch the edges all around and roll everything out again, then place on the hot tava/griddle and cook them gently, turning once or twice.
Here's a website that tells you loads more:-
http://www.indianfoodsco.com/Recipes/breads.ht m