Lubricants and greases are both substances that reduce friction between moving parts, but they differ in consistency: lubricants are typically liquids, while greases are semi-solid, made by mixing a liquid lubricant with a thickener. This difference in consistency determines their best applications, with liquid lubricants suitable for high-speed, high-temperature machinery, and greases ideal for infrequent lubrication or where a lubricant needs to stay in place and act as a seal.