Yin and yang are the opposite forces in taoist thought. Yin is the stable, unmoving, hidden aspect of things; yang is the changing, moving, revealing aspect.

Now a days the yoga now practiced in the west is yang yoga, which is an active practice that primarily focuses on movement and muscular contraction. 

Yin Yoga soft practice focuses on the connective tissues, tendons and ligaments. These are less elastic than the muscles, so in order for them to stretch they respond to a more slow and steady load. If you gently stretch connective tissue by holding a yin pose for a long time, the body will respond by making them a little longer and stronger.

This practice is perfect to promote more flexibility in areas such as hips, pelvis and lower spine. 

The main difference about these postures is the time one holds the pose, that can be between 5, 10 or more minutes.

Most of the asanas in yin yoga are in seated position like: baddha konasana, janu sirsasana, halasana, shoelace pose, sleeping swan, etc.

The most important benefit about yin yoga is to be able to ease into meditation in a comfortable seated position, and enhances overall your yoga practice and most importantly stimulates the flow of qi and prana through the connective tissue.

Here a link for yin yoga asanas: http://www.yinyoga.com/ys2_2.0_yinyoga_asanas.php