Different for girls then guys and different for age
Young girls (18-27ish) usually mistake sex and love so one night stands could be a "If I sleep with him eventually a relationship will develop" or a take charge method like "I do what I want (snap snap snap)".

Older women (30-50) a one night stand would be a lot about just needing to get some and not really concerned about the reason why or with whom or it could be for an emotional purging after a bad breakup or divorce. Getting back on the ahem, horse so to speak.

Young men (18-29) for fun. Euphemisms galore such as "getting some town bronze" "sowing wild oates", "getting seasoned", etc. etc. Young guys generally just want sex with as many people as are willing to let them. (generalization not a rule)

Older men (30-50)...no idea.