The act of sexual intercourse between two or more organisms. In humans commonly used for pleasure, with other species used specifically for reproductive purposes.
In humans, a man and a woman (or even same sex e.g. man and man) get together and begin to kiss passionately. Things get more steamy and the man and woman begin to lose clothing, either by the other person or themselves removing it.
The man, who by this point has an erect (hard) penis, and the woman, who by this point has a lubricated (wet) vagina begin to fondle eachothers bodies ('heavy petting') with either their hands, mouths or other objects (e.g. dildo, vibrator) until they both decide they're ready for penetration.
This can be done with the woman or man on top, in various positions.
With the woman on top, the woman stradles
herself at the mans crotch with her hands placed onto his stomach and slowly slides his hard penis into her wet vagina. At this point the man can be fondling her breasts as she slowly slides up and down his penis gradually getting faster. This continues until orgasm for both parties is reached, where the male can ejaculate either internally or externally.
With the man on top, the woman spreads her legs (or has the man do it for her) and either holds her legs high up (for deeper penetration) or at her side for the man to slide his penis into her. Eye-contact and closeness are positives of this position. The man then slides in and out of her, again getting faster until orgasm of both parties is reached and the man can ejaculate either internally or externally.