Custom Search Engine reflects your knowledge and interests. Specify the websites that you want searched – and integrate the search box and results into your own website. Google Custom Search Engines can be used in conjunction with the Google AJAX Search API, integrating custom search engines along with blog, video, and news search.
In defense, AJAX search API displays the combination of Google AJAX Search API and a CSE. This is featured with four CSEs on one page- product reviews, price comparisons, forums/message board and hopping. AJAX is powered with blog web and news search, also a strip of related videos.
To add to it, Matt Cutts advices:
“If you haven’t seen the latest examples of the AJAX search API, it’s worth a looksee. My favorite is the mash-up of search + a spreadsheet + mapping on this page. You could also use that page to generate phone leads, for example.”