Link units display a list of topics that are relevant to the content of your page.
Link units are closely targeted to the interests of your users. Because users directly interact with the ad unit, they may be more interested in the ads they eventually see.
AdSense publishers are paid for clicks on the ads that are linked from link unit topics, not for clicks on the initial topics themselves. The ads on the linked page are pay-per-click Google ads similar to those shown in regular AdSense ad units.