A roach is the last bit of a joint OR it is a piece of rolled up paper or cardboad you put at the the mouthpiece end of the joint (or blunt) to help it draw better. I don't really know anyone who uses the the draw helping roaches, but I know they exist somewhere. You can roll it even if its old. It still has the thc in it, which is the stuff that gets you high. It will be a little harsher, and burn a little faster, but thats about it. Tobacco is a preferance thing. Again don't know anyone who actually mixes tabacco with weed, but I hear there are some (maybe a Europe thing?). I love blunts because they have just the right amount of tobacco without needlessly destroying my lungs. You stop smoking the joint, when its too hot to hold, and/or smoke (it will burn your throat even more, because the smoke doesn't have time to cool, and it will also burn your lips). There are various traditions for what to do with the leftover roach (being the last little bit of the joint/blunt). Some people like to swallow it, (making sure that the joint/blunt is extinquished, others save it for grandparent joints (where you combine lots of roaches to make a super resinated joint. Others just toss the joint like a cigarette butt. Anyways, have fun with the weed man, and take a hit for me.