Elastic thread is ideal for ruffling or gathering. You can use it for hand or machine sewing.
To make it work properly in a sewing machine, you must wind it onto the machine bobbin by hand, being careful not to stretch it while winding. Then, when you're sewing, make sure you hold the fabric taut; you'll need a hand on the fabric that's already been sewn, stretching the elastic you've already sewn. (Seamwork has an excellent guide for this).
60% latex, 40% polyester.