Camping is popular in the United States, with many thousands of campgrounds scattered around the country, especially in the more scenic areas near its national parks and other wilderness areas. Car camping and RV camping are most common, and wilderness camping is somewhat popular in the more sparsely populated western states.
Campgrounds may be privately owned or government owned. Privately owned campgrounds generally cost more and offer more amenities and services. Government owned campgrounds generally cost less and offer fewer amenities and services. "Dispersed camping" at ad hoc or undeveloped camp sites is also allowed on many rural government-owned lands, generally at no charge but subject to restrictions on stay length and other uses.
There is much variation in how you claim a site. Popular campgrounds generally recommend advance reservations, and in some cases require them for all campers. The most popular campgrounds often fill up weeks or months in advance, with reservations for each site opening a certain number of days in advance.
Less popular campgrounds are generally "first come, first serve", meaning that the first person to park and/or set up at the site for the night gets it, and they may occupy that site as long as desired, up to any maximum number of days set by the campground rules (often 14 days). These campground usually require registration on site, during which you will get a tag to display at your campsite to indicate you have paid for it. Privately owned campgrounds are less likely to have maximum stay restrictions.