A cyst is a small sac that can grow on the skin, organs, and other body tissues. There are hundreds of different types of cysts, most of which are harmless.In some rare cases, however, these structures can be a sign of a more serious condition, such as a hormonal imbalance or cancer. cyst is a small sac that can grow on the skin, organs, and other body tissues. There are hundreds of different types of cysts, most of which are harmless.