No ad units and link units are not the same . link units will not display video ads and image or details . They display only links to different pages. 
You are allowed 3 adunits, 3 link units and 2 search per page

An Ad Unit is a box which display Ads, so search and links are not considered Ad units, because they require some action before becoming Ads (no body will be charged money if you click a link unit only or if you type a keyword for a search box, only when you click and Ad on the resultant page it will be charged).