Quickie sex, quickies or "having a quickie", all refer to a brief or spontaneous episode of sexual activity, with the couple finishing it in a very short amount of time.
A quickie does not require full copulation, it may be limited to oral sex or mutual masturbation. Generally a couple will skip foreplay as the urgency of the situation dictates a speedy finish.
Quickie sex stories are also referred to as short sex stories, where the plot is normally quite scant, and the characters are thrust into the action almost from the word go.