A classified is a marketplace listing — something for sale, housing, a job opening, a service offered. Classifieds are community content: free for anyone in your area to read, like the classifieds section of a paper always was.
The path
Open the classified form. Choose the category that fits, describe what you're listing, set a price if there is one, and add photos. Your listing appears in the community feed for your area.
When it sells or fills, edit it from My Content to mark it or take it down.
Note: a classified is a listing in the commons, not an advertisement — if you want to promote your business on journalists' feeds, that's advertising, which works differently.