Classified advertising is usually textually based and can consist of as little as the type of item being sold, (i.e., "Musical Instruments") and a telephone number to call for more information ("call 555-7777"). It can also have much more detail, such as name to contact, address to contact or visit, a detailed description of the product or products ("guitar and amplifier, Fender and Marshall" as opposed to "musical instruments", "Vintage Ludwig Drum Set" as opposed to "drum set"). There are generally no pictures or other graphics within the advertisement, although sometimes a logo may be used. Classified advertising is called such because it is generally grouped within the publication under headings classifying the product or service being offered (headings such as Musical Instruments For Sale Musical Equipment For Rent, etc.) and is grouped entirely in a distinct section of the periodical, which makes it distinct from display advertising, which often contains graphics or other art work and which is more typically distributed throughout a publication adjacent to editorial content. A hybrid of the two forms — classified display advertising — may often be found, in which categorized advertisements with larger amounts of graphical detail can be found among the text listings of a classified advertising section in a publication. Business opportunities often use classifieds to sell their services, usually employing 1-800 numbers. Classified ads are also among the tools used by many companies in recruitment for musicians available, musicians wanted and music job opportunities.

In recent years the term "classified advertising" or "classified ads" has expanded from merely the sense of print advertisements in periodicals to include similar types of advertising on computer services, the Internet (online) radio, and even television, particularly cable television but occasionally broadcast television as well, typically very early in the morning.

Like most forms of printed media, the classified ad has found its way to the Internet. Printed classified ads are typically just a few column lines in length, and they are often filled with abbreviations to save space and money. Internet classified ads do not typically use per-line pricing models, so they tend to be longer. They are also more readily searchable unlike their offline brethren, and tend to be local or global classifieds with a great sense of urgency because of their daily structure. Because of their self-policing nature and low cost structures, some classified ad companies offer free online classified ads. MBL was one of the first online musical instruments and musicians classified sites, and is currently one of the largest. There are also country-specific classified sites. There are a number of agencies throughout the world that have made a business out of the classified advertising industry. For example Wide Area Classifieds has created a classified network where people can place ads in papers across the US.

In 2003, the market for classified ads in the United States was $15.9 billion (newspapers), $14.1 billion (online) according to market researcher Classified Intelligence. The worldwide market for classified ads in 2003 was estimated at over $100 billion.