Classified advertising is a form of advertising which is particularly common in newspapers, online and other periodicals. Popular Online classified site is http://www.craigslist.org/about/sites In classified submission , you need title and description and the relevant category. you can also add the image of your services and url of your website.
Classified Ad is one type of Marketing it may be free or paid listing and also it may be locally and international base. Classified Ads visitor generally low but targeted like if you want to buy a computer then you only browse on that category where you can able to get the information about the seller of computer so classified ads some sort of may be helpfull specially if you doing your business locally. Recently you may find that search engine specially google like this type of classified Ads site. how ? Ok let try a experiment. I am from Kolkata doing job of SEO training. I have added my site or my business proposal in some of free advertisement sites. Now type in google "SEO training in Kolkata" you will find in first page some of most popular free classified Ad site, like quicker.com and ibibo.com etc. So if you target customers locally then classified ad may help you. But I am not sure how much it is related to real term of Search Engine Optimization.
classified ads are free advertisements as well as paid services also.Some classified sites have good Alexa Rank like gumtree.com, if you post ad like this type of sites you will get more traffic.
Classified is a type of ad like we post ad a news in news paper as the same is classified.We can get the traffic from classified and if we post our classified in the particular area from where we want to get business then classified is the best way.Some classifieds are free and some are paid for time duration.Here find the classifieds : http://www.olx.com/ http://www.locanto.com/