Term created by Abbie Hoffman and Jerry Rubin in the mid-1960s to refer to "members" of the Youth International Party (YIP!). The YIP! was dedicated to merging New Left activism and the hippie counterculture to create a revolution that would be both personal and political--as well as fun. Yippies tended to gather in large cities, particularly in Manhattan's Lower East Side, where Rubin and Hoffman both lived during the 1960s. Yippies rejected all -isms, including socialism and anarchism, in favor of the motto of "Do your own thing"--i.e., don't conform to a specific system of belief but rather be an individual. At the same time, collective action was at the root of Yippie activism, and Yippies participated in "be-ins" (normally associated with hippies) and other collective gatherings. And although the YIP! did not promote any one -ism (and, despite Hoffman and Rubin's involvement, was a self-proclaimed "leaderless" movement), the "party" was extremely leftist, advocating social justice for all and arguing that all property--including housing, clothing, and food--should be FREE. The Yippies' most famous actions include the attack on the New York Stock Exchange (when Yippies threw money to the floor and watched as those below fought for it) and their involvement at the 1968 Democratic Convention in Chicago, at which they nominated a pig for president. Much of the Yippies' activism consisted of guerrilla street theater and symbolic acts (such as that at the NYSE) to make a point; Yippies understood the power of the media and sought press to disseminate their revolutionary messages with a pointed disinterest in the accuracy of the stories told about them. Since the term is rooted in a particular historical moment, calling anyone a "Yippie" today would probably be inaccurate.
Abbie Hoffman's Revolution for the Hell of It! is an excellent source for more information on the Yippies.