Classified advertising is different from standard advertising in that it allows not only corporate entities or companies, but to individuals as well, to actually be able to reach out to an audience and solicit for business. It is much more affordable to have a classified advertisement in the next issue of newspapers, than have to produce a television commercial, for individuals.
Classified advertising (or classified ads) usually contains just a few lines of text containing details like the nature of a business and contact information. Different from other types of advertising, classified ads are essentially textual and do not contain any pictures.
With the invention of the Internet and World Wide Web, classified advertising, like many other printed media, has progressed online.
Just to compare two points between printed and online media:
Online classifieds ads, unlike conventional printed ones, do not have space constraint. Typically, online classified sites now provide a full-page (the size of your monitor screen) display of ads for a standard fee, compared to printed ads, which has costs directly proportional to the size of the ad.
Whereas printed classified ads (via newspapers, mainly) are only reachable to people who actually buy the newspapers, and more often than not, are only in a small region, online classified ads are available to the entire world; just as long there is an Internet connection, of course.
Popular sites like Craiglist and Ablewise.com provide very easy-to-use systems for posting classified ads. With massive traffic to their sites everyday, users are able to sell their products within days after posting their ads.