Banner ads rely on images rather than text and are a popular form of online advertising. The purpose of banner advertising is to promote a brand and/or get visitors from the host's site to go to the advertiser's site. Banner advertising, also called display advertising, consists of static or animated images or media and is usually placed in high-visibility areas on high-traffic websites. A banner ad is attractive because it can help create brand awareness, generate leads, and retarget an audience (such as giving a visitor a chance to sign up for a newsletter or free trial before clicking away). Traditional banner ads like the example above have expanded to include other forms, such as Facebook ads and Instagram banner ads. Facebook and Twitter account for nearly a third of online display ad spending. The trend in online advertising has seen digital display ad spending surpass search-based advertising in recent years. As of 2018, nearly one-fifth of all online ad spending goes to digital display ads, such as banner ads and similar ads. Ad networks are responsible for matching advertisers with websites that want to sell ads. They track available ad space and match it to advertiser demand. The technology that enables ad networks to do this is a central ad server, which selects specific ads tailored to the site visitor based on keywords from the visitor's search and site viewing behavior or based on the overall context of the host website's content