The term "Performance Art" got its start in the 1960s in the United States. It was originally used to describe any live artistic event that included poets, musicians, filmmakers, etc. - in addition to visual artists. If you weren't around during the 1960s, you missed a vast array of "Happenings," "Events" and Fluxus "concerts," to name just a few of the descriptive words that were used.
It's worth noting that, even though we're referencing the 1960s here, there were earlier precedents for Performance Art. The live performances of the Dadaists, in particular, meshed poetry and the visual arts. The German Bauhaus, founded in 1919, included a theater workshop to explore relationships between space, sound, and light. The Black Mountain College (founded [in the United States] by Bauhaus instructors exiled by the Nazi Party), continued incorporating theatrical studies with the visual arts - a good 20 years before the 1960s Happenings happened. You may also have heard of "Beatniks" - stereotypically: cigarette-smoking, sunglasses and black-beret-wearing, poetry-spouting coffeehouse frequenters of the late 1950s and early 1960s. Though the term hadn't yet been coined, all of these were forerunners of Performance Art.
By 1970, Performance Art was a global term, and its definition a bit more specific. "Performance Art" meant that it was live, and it was art, not theater. Performance Art also meant that it was art that could not be bought, sold or traded as a commodity. Actually, the latter sentence is of major importance. Performance artists saw (and see) the movement as a means of taking their art directly to a public forum, thus completely eliminating the need for galleries, agents, brokers, tax accountants and any other aspect of capitalism. It's a sort of social commentary on the purity of art, you see.
In addition to visual artists, poets, musicians, and filmmakers, Performance Art in the 1970s now encompassed dance (song and dance, yes, but don't forget it's not "theater"). Sometimes all of the above will be included in a performance "piece" (you just never know). Since Performance Art is live, no two performances are ever exactly the same.
The 1970s also saw the heyday of "Body Art" (an offshoot of Performance Art), which began in the 1960s. In Body Art, the artist's own flesh (or the flesh of others) is the canvas. Body Art can range from covering volunteers with blue paint and then having them writhe on a canvas, to self-mutilation in front of an audience. (Body Art is often disturbing, as you may well imagine.)
Additionally, the 1970s saw the rise of the autobiography being incorporated into a performance piece. This kind of story-telling is much more entertaining to most people than, say, seeing someone shot with a gun. (This actually happened, in a Body Art piece, in Venice, California, in 1971.) The autobiographical pieces are also a great platform for presenting one's views on social causes or issues.
Since the beginning of the 1980s, Performance Art has increasingly incorporated technological media into pieces - mainly because we have acquired exponential amounts of new technology. Recently, in fact, an 80's pop musician made the news for Performance Art pieces which use a Microsoft® PowerPoint presentation as the crux of the performance. Where Performance Art goes from here is only a matter of combining technology and imagination. In other words, there are no foreseeable boundaries for Performance Art.
Source: Rosalee Goldberg: 'Performance Art: Developments from the 1960s', The Grove Dictionary of Art Online, (Oxford University Press) http://www.oxfordartonline.com/public/