Display ad campaigns utilize an image as the central focus of the ad. But the difference between display ads and search ads is more fundamental: their placement, function, and reach.
Billboards are different from The Yellow Pages
Display ads contain images and are shown on ad networks. Display ads are shown in places and at times when the targeted audience may not be expecting them. Display ads function as advertisements, whereas search ads are a form of direct marketing offer.
Search ads are shown on search engine results pages, in response to a search engine query. Search ads function more like a lookup directory – a modern ‘Yellow pages” listing.
A classic reference on advertising is Claude Hopkins’ The Science of Advertising – published in 1923 and just as applicable today.