Climate
Chaco, like most of northwestern New Mexico, has a high desert climate with four distinct seasons. Spring is dry and windy, with high temperatures rising rapidly from an average of 60 degrees Fahrenheit or so in March to over 80 F in June. Summer is hot, with highs frequently above 90 F, and with much of the average year's precipitation falling in isolated, brief but violent thunderstorms. However, it's the legendary "dry heat" that doesn't feel nearly as hot as in regions with higher humidity. Cooling starts in August and leads to a dry, temperate fall season that is usually a good time to visit Chaco. Winter is also pleasant with highs around 50 F and clear skies, although there are usually three or four frontal storms each winter that bring snow, usually in small quantities but with the occasional major snowstorm. The remoteness of the park is such that it's a good idea to check a weather forecast before visiting in the winter; Farmington, about 60 miles away, usually experiences similar weather, and its current conditions and forecast are updated regularly. If there's significant snow say 6" or more in the forecast, it's wise to defer your trip unless you're particularly well prepared for snowy roads.
Flora and fauna
Chaco has high-desert flora, with sagebrush, cactus, etc., interspersed with small bits of piñon/juniper scrub forest, the latter primarily on the mesa tops. Chaco receives less precipitation average 8 inches per year than some other parts of New Mexico at similar latitude and elevation, and consequently does not have the coniferous forest of some areas to its east. Plant life is generally sparse.
Animal life too is not as abundant as elsewhere in the state, but wildlife encounters are still reasonably frequent. The largest animal you're likely to see is the ubiquitous coyote, although you may also see a deer or two. Elk and antelope are present in the region but rarely encountered in the park. Smaller carnivores such as bobcat, badger, fox and two species of skunk are sometimes seen, and rodents are locally abundant, with a few prairie-dog towns in the park. Small colonies of bats are present during the summer.
The shortage of water reduces the prevalence of bird life as well, but Chaco is still an island of at least relative avian abundance and diversity in this region. With luck you'll see a roadrunner or two, but don't count on it; they're rare here. The usual assortment of medium to large hawks Cooper's hawk and kestrel are fairly common, owls more often heard than seen, vultures and raven are present, if less abundant than in the mountains to the east. There are reasonably large populations of smaller birds, with warblers, sparrows, house finches, etc., common. Three species of hummingbirds are at hand, and one of the treats of late summer is watching the tiny but incredibly pugnacious rufous hummingbirds chasing off the larger, mellower black-chinned hummers that compete with the rufous for habitat. It's like watching a World War I aerial dogfight in miniature. Rufous hummers are rare in the park, however, and you'll be lucky to get this visual treat.
Western prairie rattlesnakes are occasionally seen in the backcountry, but you're much more likely to see various lizards scurrying along the tops of restored walls of structures, with skinks being fairly abundant.
History
The Chaco dwellings were built and occupied primarily between about 850 and 1250 AD, during which time they were the hub of a remarkable network of transportation routes, many of which survive today as the "roads" of Chaco. They fell into disuse after 1300, probably due to climate change, although descendants of the Chacoans and other tribes remained aware of the ancestral pueblos. The present park was one of the first units of the National Park System to be formed specifically to protect archaeological resources, being first formed as Chaco Canyon National Monument in 1907 shortly after Mesa Verde National Park, which also started as a national monument rather than a park. The monument achieved national-park status in 1980 and Pueblo Bonito became a UNESCO World Heritage Site in 1987.
Landscape
Chaco is in canyon/mesa country, like most of northwestern New Mexico, but the topography in the park isn't as spectacular as in some other areas of the state; the visitor center, in the canyon bottom, is at an elevation of about 6200 feet, and the surrounding mesas rise only about 400 feet above this level. The canyon is wide and open for most of its length, unlike the narrow "slot canyons" to the northwest in Utah. This, of course, is why there's a park here: the openness of the canyon provided enough room to build the pueblos and grow crops. Note that this elevation is high enough to challenge the lungs of the visitor freshly up from sea level. You may find yourself a bit short of breath when hiking, so if you're going to be there overnight, it's recommended that you do the "easy" trails along the main loop first to get acclimatized somewhat, then the longer, more vertical backcountry trails on your second day.
Most of the rock comprising the mesas as well as the canyon floor is sedimentary, and consequently tends toward flat bedding planes that are interrupted by relatively few faults and folds. As a result, one of the hiking challenges is that the transition between individual geological formations along the canyon walls can be steep. This is incentive to stay on the constructed trails in the backcountry; you'll want to do that anyway, as off-trail hiking is generally prohibited and the prohibition is enforced.
One of the signature features of Chaco is Fajada Butte near the south entrance. This narrow, steep-walled butte rises about 400 feet above the canyon and is notable for artifacts including the "Sun Dagger," interpreted by some as an astronomical observatory/clock used by the Chacoans to keep track of annual progress toward the summer solstice. Fajada Butte is no longer open to hikers, but a roadside turnout near the south entrance leads to a viewpoint from which it can be seen and photographed to good advantage.