1985 advert from the UK for Yellow Pages of a Son trying to find a train for his Dad.
The term Yellow Pages refers to a telephone directory of businesses, categorized according to the product or service provided. As the name suggests, such directories are usually printed on yellow paper, as opposed to white pages for non-commercial listings. The traditional term Yellow Pages is now also applied to online directories of businesses.
The name and concept of “Yellow Pages” came about in 1883, when a printer in Cheyenne, Wyoming working on a regular telephone directory ran out of white paper and used yellow paper instead. In 1886 Reuben H. Donnelley created the first official yellow pages directory, inventing an industry. 
Today, the expression Yellow Pages is used globally, in both English-speaking and non-English speaking countries. In the United States, it refers to the category, while in some other countries it is a registered name and therefore a proper noun.
Yellow Pages (YP) are a print directory which provides an alphabetical listing of businesses within a specific geographical area (e.g., Greater Chicago), which are segregated under headings for similar types of businesses (e.g., Plumbers). Traditionally these directories have been published by the local phone company, but due to the highly profitable nature of the business there are numerous independent directory publishers. Some YP publishers focus on a particular demographic (e.g., Christian Yellow Pages or Business Pages).
Yellow Pages directories are usually published annually, and distributed for free to all residences and businesses within a given coverage area. The majority of listings are plain and in small black text, usually in the Bell Gothic or Bell Centennial typefaces. The YP publishers generate profit by selling advertising space or listings under each heading. Advertising may be sold by a direct sales force or by approved agencies (CMR’s). Available advertising space varies among publishers and ranges from bold names up to four color twin page ads (“double trucks”). Advertising rates typically increase every year regardless of distribution or usage fluctuations.
In the United States, the predominant yellow pages are the RH Donnelley’s DEX, Yellowbook, the AT&T Real Yellow Pages, and the Verizon Superpages.
In some countries, the familiar “walking fingers” logo is not protected as a trademark and may be used by anyone. This logo is used in varying forms by almost every YP publisher; however, there are companies that use it to imitate mainstream publishers. In Israel, the directory, although using the Yellow Pages logo, is called “Golden Pages”.
Business listings used for publication are obtained by several methods. Local phone companies that publish YP directories rely on their own customer lists and include business listings that are provided by phone service providers (CLEC’s). Business owners that utilize phone services other than the local phone company (typically a Bell Company) should make certain that their information has been sent to the publisher for printing in upcoming directories.
Advertising in YP directories requires payment in full prior to printing or may be billed monthly over the life of the contract, which is usually 12 months. Typically, a sales representative will assist the customer in creating their ad design and provides a Proof Copy for review and approval. Advertisers should be aware that many contracts have automatic renewal clauses and require action on the part of the advertiser to end future billing.
Yellow Pages print usage is reported to be declining with both advertisers and shoppers increasingly turning to Internet search engines and online directories. According to a study by Knowledge Networks/SRI, in 2007, print Yellow Pages were referenced 13.4 billion times, while Internet Yellow Pages references increased to 3.8 billion, up from 2006’s 3.3 billion online searches. As a result most YP publishers have attempted to create online versions of their print directories. These online versions are referred to as IYP or Internet Yellow Pages. Independent ad agencies or Internet marketing consultants can assist business owners in determining sound opportunities for YP advertising and provide objective information on usage, possession and preferences.
Info taken from Wikipedia