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.

Traditional billboards

Billboards are typically large wooden signs, with the larger ones typically 48'x14' or
24'x12' (width x height). The display is painted or printed on a vinyl sheet which is glued
onto the board. Smaller 22'x10' and 20'6"x9' billboards display a series of thirty or twenty
four printed posters respectively to make up the sign. This format is cheaper to produce but
has less visual impact.

Mobile billboards

Billboards can also be made mobile, either by mounting a traditional billboard onto a trailer or flatbed truck, or by covering an entire vehicle in a "wrap" image. This is sometimes used in
bus advertising, though it is more common to mount smaller "boards" on those vehicles.

Advertising style

Billboard advertisements are designed to catch a person's attention and create a
memorable impression very quickly, leaving the reader thinking about the advertisement
after they have driven past it. They have to be readable in a very short time because they
are usually read while being passed at high speeds. Thus there are usually only a few words,
in large print, and a humorous or arresting image in brilliant color.

Source: http://en.wikipedia.org/wiki/Billboard_advertising