A billboard or hoarding is a large outdoor signboard, usually wooden, found in places with high traffic such as cities, roads, motorways and highways. Billboards show large advertisements aimed at passing pedestrians and drivers. The vast majority of billboards are rented to advertisers rather than owned by them.

Typically showing large, witty slogans splashed with distinctive color pictures, billboards line the highways and are placed on the sides of buildings, peddling products and getting out messages. Billboards originally existed alongside and later largely replaced advertisements painted directly onto the sides of buildings or designed into roofs in shingle patterns.

New billboards are being produced that are entirely digitized (using projection and similar techniques), allowing animations and completely rotating advertisements. Even holographic billboards are in use in some places.

Interaction is an emerging theme in electronic billboards, with Britain at the forefront: in Piccadilly Circus the Coca-Cola billboard responds to the weather and responds with an animated wave when passersby wave at it.* London movie theatres are experimenting with billboards which contain an embedded computer chip which can interact with the web browser found in many cell phones to provide more information on the subject of the advertisement.* In the spring of 2004 in Times Square in New York City, a Yahoo! Autos promotion displayed on an LED billboard allowed one to call a phone number with a cell phone and play a two-person racing game where the cars appeared on the billboard.* There are also upcoming billboard technologies that will synchronize with advertisements on radio stations. Shinjuku in Tokyo, Japan, is famous for its large digital billboards.

Many groups such as Scenic America have complained that billboards on highways cause too much clearing of trees and intrude on the surrounding landscape, with billboards' bright colors, lights and large fonts making it hard to focus on anything else. Other groups believe that billboards and advertising in general contribute negatively to the mental climate of a culture by promoting products as providing feelings of completeness, wellness and popularity to motivate purchase. One focal point for this sentiment would be the magazine AdBusters, which will often showcase politically motivated billboard and other advertising vandalism, called culture jamming.

In 2000, rooftops in Athens had grown so thick with billboards that it was very difficult to see its famous architecture. In preparation for the 2004 Summer Olympics, the city embarked on a successful four-year project demolishing the majority of rooftop billboards to beautify the city for the tourists the games will bring, overcoming resistance from advertisers and building owners. These billboards were for the most part illegal, but had been ignored up to then.

Most highway signs exist to advertise local restaurants and shops in the miles to come and are crucial to drawing business in small towns that no one would stop at otherwise. One illuminating example is Wall Drug, which in 1931 put up billboards advertising "free ice water" and the town of Wall, South Dakota as it is known today was essentially built around the 20,000 customers per day those billboards were bringing in as of 1981. Some signs were even placed in locations great distances away, with slogans such as "only 827 miles to Wall Drug, with FREE ice water." In some areas the signs were so dense that one sign almost immediately followed the last. This situation changed after the Highway Beautification Act was passed; the proliferation of Wall Drug billboards is sometimes cited as one of the reasons the bill was passed.

Prior to 1999, billboards were a major venue of cigarette advertising; 10% of Michigan billboards advertise alcohol and tobacco, according to the Detroit Free Press.* This is particularly true in countries where tobacco advertisements are not allowed in other media. For example in the U.S. tobacco advertising was banned on radio and television in 1971, leaving billboards and magazines as some of the last places tobacco could be advertised. Billboards made the news in America when, in the tobacco settlement of 1999, all cigarette billboards were replaced with anti-smoking messages. In a parody of the Marlboro Man, some billboards depicted cowboys riding on ranches with slogans like "Bob, I miss my lung."

Likely the best-known of the tobacco advertising boards were those for "Mail Pouch" chewing tobacco in the United States during the first half of the 20th century (pictured above). The company agreed to paint two or three sides of a farmer's barn any color he chose in exchange for painting their advert on the one or two sides of the structure facing the road. The company has long since abandoned this form of advertising, and none of these adverts have been painted in many years, but some are still viewable on various rural highways around the country, though less of them each year, as they are continually weathering, being overpainted or simply torn down.