Party poppers contain a small explosive charge, usually in the bit you hold. This is a small tube of paper with the string rolled into the middle. The inside of the paper tube is coated with a friction sensitive explosive composition that usually contains potassium chlorate and phosporous (the same as toy caps). When you pull the string from the middle of the tube, friction sets off the explosive. Above the charge is a cardboard disc, then streamers, then the card disc in the end of the popper that holds it all in. The disc above the charge helps to prevent fire transferring to the streamers, also the streamers are fireproofed (sometimes by being soaked in alum solution or similar). The explosion ejects the contents and presto, a millisecond of mild amusement.