Classified advertising is usually textually based and can consist of as little as the type of item being sold, (i.e., "Clothing") 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 ("pants and sweaters, size 10" as opposed to "clothing", "red 1996 Pontiac Grand Prix" as opposed to "automobile"). 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 Accounting, Automobiles, Clothing, Farm Produce, For Sale, 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 available 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, radio, and even television, particularly cable television but occasionally broadcast television as well, typically very early in the morning hours.

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 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 classifieds with a great sense of urgency because of their daily structure. Because of their self-policing nature and low cost structures, some companies offer free online classified ads such as Craigslist in the US, and Kijiji and Gumtree internationally. Companies such as Khichdee.com have begun to offer search across thousands of classifieds that appear in various media in India and South Asia. Some companies focus mainly on urban centers, while others blanket all areas by using zip codes. Craigslist was one of the first online classified sites, and is currently one of the largest. AdverShow.com is a community website based in Northern Michigan that charges a minimal fee for posting ads with a large description and up to 5 pictures, and donates 50% of the cost to local charities that the user defines. 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.

As the online classified advertising sector develops, there is an increasing emphasis toward specialization. Like search engines, classified websites are often vertical in nature with sites providing advertising platforms for niche markets of buyers of sellers.