dtr = double treble crochet. 

dtr = double treble crochet.  Here's a link to a video tutorial  http://www.nexstitch.com/v_double_treble_dec.htmlFor a dtr, you would wrap the yarn around your hook 3 times before you insert your hook into a chain or stitch, then you yarn over pull yarn through yarn over and repeat 3 times.  I would assume that you would decrease like you do any other crochet decrease which is that you would not complete the stitch by working the last two loops on your hook.  Instead, leave them there and begin another dtr, and when you come to the last yarn over draw through 2 loops, you would draw it through 4 loops thereby making 2 stitches into one.