Lubricants are used in machines such as automotive engines to reduce friction.
Natural lubricants in the human body
In the human body, there are natural lubricants, such as the slippery fluid secreted from the walls of the vagina during sexual arousal (see also vaginal lubrication). Another is Cowper's fluid (pre-cum), a clear fluid that is issued from a man's penis prior to ejaculation when he is sexually aroused.
Personal lubricants are products which are designed for use as lubricants for vaginal sex, anal sex, and masturbation - including vaginal and anal sex toys such as vibrators, dildos, anal beads and butt plugs.
They can be water-based, oil-based, or silicone-based. Whenever a condom is used, oil-based lubricants must not be used because they chemically damage the material of the condom, making it break. This includes vaseline and body lotions. Water-based lubricants should be used instead. Even when no condom is used, oil-based lubricants should never be used for vaginal sex due to risk of local bacterial infection.