Any steroid-based alcohol having a hydrocarbon (aliphatic) side-chain of 8-10 carbons at the 17-beta position and a hydroxyl group (-OH) at the 3-beta position (that's why it's an alcohol). Cholesterol is a sterol. Because of its hydrophilic property at the -OH end and hydrophobicity at the hydrocarbon side chain, it can be incorporated into the lipid bilayers of the cytoplasmic membrane. However, sterols only exist in the cytoplasmic membranes of eukaryotes while in prokaryotes, virtually none have sterols in their membranes except mycoplasmas, a group of cell wall-less bacteria.