A Yin Yoga class is different in that it usually consists of only of a few poses. Each pose is held for an extended period.
These classes are designed to get into stretching and strengthening the tendons and ligaments. This is different from an active Hatha practice which gets more into stretching and strengthening the muscles in comparison. For this reason, a regular Yin practice makes both an ideal starting point and compliment to any practice you might already have.