A billboard consists of a very large (10 feet by 20 feet or larger) sign which carries a photograph or image related to advertising a product or service.
Arguably, billboards started from two otherwise innocuous methods. The first was Burma-Shave's use of five small signs along roads giving a poem. The second was the practice of some companies agreeing to pay to paint a farmer's barn in heavily travelled areas in exchange for being allowed to paint their ad on the side of the barn that faced a major highway. Sometimes the farmer got additional payments in addition to the free paint job.
Billboards spread everywhere, especially along interstate highways, and are now being revamped in the digital age. Now there are digital billboards that change images every few seconds. A good example of these are in New York's Times Square.
This is completely unrelated to Billboard magazine, which reports trends in the record industry. It used to be related, however, which is why it's Billboard and not Record.