The waterfall system consists of about 270 falls along 2.7 kilometres (1.67 miles) of the Iguazu River. Some of the individual falls are up to 82 metres (269 feet) in height, though the majority are about 64 metres (210 feet). The Garganta del Diablo or Devil's Throat (Garganta do Diabo in Portuguese), a U-shaped 150-metre-wide and 700-metre-long (490 by 2300 feet) cliff, is the most impressive of all, and marks the border between Argentina and Brazil. Most of the falls are within Argentine territory, but from the Brazilian side a more panoramic view of the Garganta del Diablo is obtained. Many islands split up the falls, including several large ones. About 900 metres of the 2.7-kilometre length does not have water flowing over it. The first European to find the falls was the Spanish Conquistador Álvar Núñez Cabeza de Vaca, after whom one of the falls in the Argentine side is named.

The falls are shared by the Iguazú National Park (Argentina) and Iguaçu National Park (Brazil). These parks were designated UNESCO World Heritage Sites in 1984 and 1986, respectively.

The name Iguazu comes from the Guarani or Tupi words y (water) and guasú (big). Legend has it that a god planned to marry a beautiful aborigine named Naipí, who fled with her mortal lover Tarobá in a canoe. In rage, the god sliced the river creating the waterfalls, condemning the lovers to an eternal fall.

There are two main towns on either side of the falls: Foz do Iguaçu in the Brazilian state of Paraná, and Puerto Iguazú in the Argentine province of Misiones. Other important tourist attractions near the falls include the Itaipu hydroelectric power plant, and the Jesuit Missions of the Guaranis in Argentina, Paraguay and Brazil.

Upon seeing Iguazu, First Lady Eleanor Roosevelt reportedly exclaimed "Poor Niagara!" Vastly larger than North America's Niagara Falls, Iguazu is rivalled only by Southern Africa's Victoria Falls which separates Zambia and Zimbabwe (this is excluding extremely large rapid-like falls such as Livingston de Chutes and Boyoma Falls). Whilst Iguazu is wider because it is split into about 270 discrete falls and large islands, Victoria is the largest curtain of water in the world, at over a mile wide and over 350 feet (108 meters) in height (in low flow Victoria is split into five by islands; in high flow it can be uninterrupted).

The water falling over Iguazu in peak flow has a surface area of about 400,000 square metres (1.3 million square feet) whilst Victoria in peak flow has a surface area of over 550,000 square metres (1.8 million square feet). By comparison, Niagara has a surface area of under 183,000 square metres (600,000 square feet). Victoria's annual peak flow is also greater than Iguazu's annual peak—9.1 million litres per second versus 6.5 million—though in times of extreme flood the two have recorded very similar maximum water discharge (well in excess of 12 million litres per second). Niagara's annual peak flow is about 2.8 million litres per second, although an all-time peak of 6.8 million has been recorded. Iguazu and Victoria fluctuate more greatly in their flow rate. Mist rises between 30 and 150 metres (100 and 500 feet) from Iguazu's Garganta do Diabo, and over 300 metres (1,000 feet) above Victoria (sometimes over 600 metres).

Iguazu, however, affords better views and walkways and its shape allows for spectacular vistas. At one point a person can stand and be surrounded by 260 degrees of waterfalls. The Garganta do Diabo has water pouring into it from three sides, which makes for an exceptional sight. Likewise, because Iguazu is split into many relatively small falls, one can view these a portion at a time. Victoria does not allow this, as it is essentially one waterfall that falls into a canyon and is too immense to appreciate at once (except from the air). Iguazu and Victoria are generally regarded as the world's most spectacular waterfalls, with people divided as to which is the more impressive.

As of July 24, 2006 a severe drought in South America had caused the river feeding the falls to become parched, reducing the amount of water flowing over the falls to be reduced to 300,000 litres (80,000 gallons) per second, down from the normal flow of 1,300,000 to 1,500,000 litres (350,000 to 400,000 gallons) per second. By early December, the flow was spectacular again, according to visiting tourists.