Located in the Northwest corner of Arizona, USA, the Grand Canyon is a really big canyon. It was formed over the last 17 million years, as the Colorado River flowed across the Colorado Plateau; it currently averages 4,000 feet (1220 m) deep, with a maximum depth of 6,000 feet (1830 m), and is about fifteen miles (24 km) across at its widest point. It is 277 miles (446 km) long, following the river.
The Grand Canyon begins at Lees Ferry and ends at Grand Wash Cliffs, although this present course is relatively new, only about five million years old*. It is contained almost completely in Grand Canyon National Park, although the Hualapai Reservation borders a long stretch of the South-West Rim.
Over four million people visit the Grand Canyon every year; 90% of these visit the South Rim of the canyon. Needless to say, it is often overcrowded, particularly in the summer months. The North Rim is both less crowded and less touristy, although it is also harder to get to (it is the 'other side' from both Flaggstaff and Grand Canyon Airport). The South Rim is also preferred because it is only 7,000 ft. in elevation, while the North Rim is about 8,000, and often has heavy snowfall in the winter months.
History
Humans have lived in and around the Grand Canyon for about 3,000 to 4,000 years. The earliest settlers we know of were the Desert Archaic people. We don't know a lot about them, but they did leave their pictographs and small twig representations of animals. Around 1,000 BCE the Desert Archaic people were replaced by (or became) the Anasazi, who gave up the hunter-gatherer way of life for a society based on agriculture. This trend was continued through the Pueblo Indians, who built larger and more permanent villages. Their adobe houses can still be found in some areas of the Grand Canyon. **
In 1540, Spaniards from Francisco Vasquez de Coronado's expedition headed into the North American Southwest, searching for the Seven Golden Cities of Cibola; the expedition discovered the Grand Canyon, but no riches, and returned to Mexico City a complete failure.
The next expedition to explore the Grand Canyon and surrounding areas was the Dominguez-Escalante Expedition of 1776, in which the Spanish Franciscan missionaries Father Silvestre Velez de Escalante and Father Francisco Dominguez set out from Santa Fe in hopes of finding their way to California. They didn't reach California, but they did cross over the Grand Canyon, mapping the area and giving their names to a number of the surrounding locations.
In 1869 Major John Wesley Powell led the first successful expedition down the length of the Grand Canyon by boating down the Colorado River. Aside from his army career, he was interested in botany, zoology, geology, and cultural anthropology. Powell had theorized that the river had existed before the canyon, and had formed the canyon as the Colorado Plateau slowly rose up; testing this theory was a large part of his motivation for this expedition. Three months later, he finally completed his trip, convinced that his theory was correct (it almost certainly was).
By the 1880s, areas near the Grand Canyon were being developed by livestock companies. In 1893 the Grand Canyon was set aside as a forest reserve. In 1901 the first Santa Fe passenger train arrived at the South Rim. The tourism trade slowly started to take off. The Grand Canyon became a national monument in 1908, and on February 26, 1919, it was designated as the seventeenth national park. (To put this in perspective, Arizona became a state in 1912).
There are a couple of dams near the canyon, although none actually in it. Glen Canyon Dam (1963) is 15 miles (24 km) upstream of Lees Ferry, creating Lake Powell. At the other end of the canyon is Hoover Dam (1936), forming Lake Mead; Lake Mead will sometimes flood part of the lower regions of the canyon. In 1969 President Lyndon B. Johnson created Marble Canyon National Monument, blocking further attempts to place dams in the Grand Canyon.
My old 1992 guide book reports that the entrance fee to Grand Canyon National Park is $10.00 per vehicle, regardless of the number of passengers; $4.00 if you come by public transportation (bus, taxi, or train). If you plan on staying in the park (or anywhere nearby, I suppose), you should make reservations as far in advance as you can; during the summer months, the Grand Canyon hotels may be booked solid six months in advance.
This information may be out of date.
* There is some debate over what exactly its older course was, and why it changed. All theories I have seen agree that the Western part of the canyon is either new, or spent a few million years dry, while the Eastern part has seen constant use for the entire life span of the canyon.
** I'd like to give more detail, but I don't have any information on the specific groups that lived in the Grand Canyon. The cultures named here (Desert Archaic, Anasazi, Pueblo), are general names for cultural trends that covered large areas (multiple states). You can read more about them in the appropriate nodes.