Happening Vs. Performance

A happening is a performance, event or situation meant to be considered as an art. Happenings take place anywhere (from basements to studio lofts and even street alley ways), are often multi-disciplinary, with a nonlinear narrative and the active participation of the audience. Key elements of happenings are planned, but artists sometimes retain room for improvisation. This new media art aspect to happenings eliminates the boundary between the artwork and its viewer. Henceforth, the interactions between the audience and the artwork makes the audience, in a sense, part of the art.
In the later sixties, perhaps due to the depiction in films of hippie culture, the term was used much less specifically to mean any gathering of interest, from a pool hall meetup or a jamming of a few young people to a beer blast or fancy formal party.

Allan Kaprow first coined the term happening in the Spring of 1957 at an art picnic at George Segal's farm to describe the art pieces that were going on. Happening first appeared in print in the Winter 1959 issue of the Rutgers University undergraduate literary magazine, Anthologist. The form was imitated and the term was adopted by artists across the U.S., Germany, and Japan. Jack Kerouac referred to Kaprow as "The Happenings man," and an ad showing a woman floating in outer space declared, "I dreamt I was in a happening in my Maidenform brassiere."
"Happenings" are very difficult to describe, in part because each one is unique and completely different from one another. One definition comes from Wardrip-Fruin and Montfort in The New Media Reader, "The term "Happening" has been used to describe many performances and events, organized by Allan Kaprow and others during the 1950s and 1960s, including a number of theatrical productions that were traditionally scripted and invited only limited audience interaction.

Or...

Performance art is art in which the actions of an individual or a group at a particular place and in a particular time constitute the work. It can happen anywhere, at any time, or for any length of time. Performance art can be any situation that involves four basic elements: time, space, the performer's body and a relationship between performer and audience. It is opposed to painting or sculpture, for example, where an object constitutes the work. Performance art traditionally involves the artist and other actors, but works like Survival Research Laboratories' pieces, utilizing robots and machines without people, may also be seen as an offshoot of performance art.
Although performance art could be said to include relatively mainstream activities such as theater, dance, music, and circus-related things like fire breathing, juggling, and gymnastics, these are normally instead known as the performing arts. Performance art is a term usually reserved to refer to a kind of usually avant-garde or conceptual art which grew out of the visual arts. Uniquely, Michel Lotito ("M. Mangetout") made performance out of eating unusual objects.

Performance art, as the term is commonly understood, began to be identified in the 1960s with the work of artists such as Yves Klein, Allan Kaprow—who coined the term Happenings—Carolee Schneemann, Hermann Nitsch, Yoko Ono, Wolf Vostell, Joseph Beuys, Barbara T. Smith, Vito Acconci, the women associated with the Feminist Studio Workshop and the Woman's Building in Los Angeles, and Chris Burden. But performance art was certainly anticipated, if not explicitly formulated, by Japan's Gutai group of the 1950s, especially in such works as Atsuko Tanaka's "Electric Dress"