Pucker is the wrinkling, uneven surface on a fabric which is the result of incorrect sewing tension of a machine, wrong thread or fabric instability. The effect of puckering can be even created by choice, through the use of several different methods such as printing with caustic soda or other chemicals which makes controlled wrinkles on it.
Purl Stitch is a type of knitting stitch. Usually made with the yarn at the front of the work, inserting the right needle into the front of a loop on the left needle from the right. Than, the yarn is catch with the right needle and bringing it through, in order to form a new loop.