Yellowstone is the world's first National Park, established by President Ulysses S. Grant on March 1, 1872. A 3,472 square mile (8,987 square km) preserve, located mostly in northwest Wyoming, but also in Idaho and Montana, Yellowstone houses one of the most diverse intact temperate ecosystems anywhere.
Yellowstone is a volcanic caldera that last erupted about 640,000 years ago. It is best known for its hot springs and geysers, most notably, Old Faithful - it is the largest concentration of geothermal features anywhere - and its large mammals - wolves, bears, and bison. As a result, it is the most popular national park, with an average of 2.8 million visitors per year.
Most of the park is wilderness. There are five entrances, and a road that makes a figure 8, essentially, in the center of the park. There are all the standard facilities for an area that attracts this many visitors - camping, hotels, resturants, visitor centers, and the like, but for the most part, the park is undeveloped. A system of reservations and high demand severly limits the number of visitors staying in the park.
During the busy season, from July 1 to the end of August, the park is crowded, on the main roads. For the rest of the year, the park is relatively quite. Alternately, one may go hiking or backpacking on any of the many trails and escape the crowds at any time of year. The trail system is open to all, though reservations are required for backcountry campsites.
Yellowstone is open year round, though the roads are closed to normal motor vehicles from the first major snowfall to the last - often October 1 - May 1. (When I was working in the area during the summer of 2001, the pass at the east entrance was closed due to a late snowfall, about 18 inches at the pass, on June 1. The pass opened halfway through the next day, so there were no major delays, but one should keep in mind that this is a place of unusal weather.) During the winter, the roads are open to snowmobiles, skiers, and backpackers.
Yellowstone is an amazing place. Go early in the season, early to mid June, plan to spend at least two or three days there. Make reservations for a place to stay ahead of time (at least MONTHS) in advance, and take your time in the park. Hike a little in the backcountry, and take it slowly. Drive slowly down the mountain roads. (Yes. This means you. It is allowable to drive the speed limit. Or less, even. Search and Rescue gets tired of cutting people out of cars.) Most of all, just enjoy the amazing diversity of the place.
Information derived from www.nps.gov/yell/ and one summer working in the Pahaska Tepee resort, at the east entrance to Yellowstone. Go. Enjoy. And if you really want to go hiking in the backcountry, let me know - I have the $25 set of maps that you will need, and I doubt I'll be using them again, before they become outdated.