Yin Yoga is a slower paced practice, with poses held at least 3 minutes (but can be taken for much longer). Holding poses for longer means working on the deeper tissues - particularly the joints. It is also a very meditative practice and the perfect complement for your Yang practice, or any other activity you might practice (i.e. strength training, running, cycling etc.)
It is really a practice that, I believe, benefits most people; we all need to slow down and take a breath in between the business of modern life!