Yin Yoga is a term that is used to describe a style of yoga that is static, held, supported poses with a particular aim of supporting joints and connective tissues. Unlike other forms of Yoga found in the West, Yin Yoga is primarily seated and lying down, with ample support.
You can expect to hold posts for 3-7 minutes, but will not be using muscles to hold yourself up. Yin Yoga is different from Restorative Yoga in a few key ways. While the poses often look the same to the outside observer (lots of props, holding poses for many minutes at a time), a practitioner notices a sense of feeling either stretch/stress in Yin, or a sense of feeling supported and nurtured in restorative.
Unless a teacher has been trained in these differences, it is common that they will confuse them and guide a practice without directing the student on how to feel, but instead what to look like.