Unlike phospholipids and fats, steroids have a fused ring structure. Although they do not resemble the other lipids, they are grouped with them because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings, and many of them, like cholesterol, have a short tail. Fats are a stored form of energy. Steroids are used for hormones or signaling molecules. Steroids such as cholesterol also decrease cell membrane fluidity.