Short answers, pulled from the story.
Improvisational theatre, also called improv or impro, is a form of theatre, often comedy, in which most or all of what is performed is unplanned and unscripted. The dialogue, action, story, and characters are created collaboratively by the performers as the scene unfolds in present time, without a prepared written script.
The earliest well-documented use of improvisational theatre in Western history is found in the Atellan Farce of 391 BC. From the 16th to the 18th centuries, commedia dell'arte performers improvised based on a broad outline in the streets of Italy.
Shortform improv consists of short scenes usually built from a predetermined game, structure, or idea and driven by an audience suggestion. Longform improv creates shows in which short scenes are often interrelated by story, characters, or themes, and may take the form of a full-length play or musical such as the Harold developed by Del Close.
Yes, and... is considered the cornerstone of improvisational technique, meaning a performer accepts another performer's offer and then adds a new offer that builds on it. Refusing an offer is called blocking, negation, or denial, which usually prevents the scene from developing.
Viola Spolin influenced the first generation of modern American improvisers, and her son Paul Sills founded The Second City, which opened on the 16th of December 1959. David Shepherd co-founded The Compass Players with Sills, and Del Close created the longform Harold format.
Applied improvisation, which began in the late 1990s, uses improv as a tool outside the performative space, including in classrooms, businesses for communication and team-work, and in psychotherapy. Tina Fey listed rules of improv that apply in the workplace, and Alan Alda explored how improv improves communication in the sciences.