Classified is the advertising of our website where as Forum Post just sharing Information about every thing. It is just like Discuss. Classified sites are the advertising sites where people can submit there various ads according to there products and services.
Classified listings is a type of ads. Through classifieds, we can promote our products. Through Forum or Community we can obtain or share our information on particular subjects, being modified on newest information both are different.
Classifieds is used for ads purpose where we can submit product or service based information.
Forum is mainly for discussion purpose it helps to gain more knowledge about a particular topic, simply we can say it is like a chat.