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.
Definition from http://en.wikipedia.org/wiki/Billboard_(advertising)