Providing different ways to access content on your website to users who are already on the site is a very important aspect of good website design. This means having a logical and clean information structure/architecture so that users can navigate easily from one page to the next in search of the information they need. While excellent site architecture is key to navigability, putting in a customized search engine (CSE) on your site is another option you should give to your users. Customized search engines are very important since most users prefer it over having to click from link to link in search of just the right page containing their desired information.
You can easily create customized search engines using Google’s Custom Search and have it search only one site, several sites, or even just specific webpages. You can also customize the look of the search bar to make sure that it goes well with the rest of the website design, and even customize the results layout so as to take advantage of rich snippets using rich results templates. Your custom search can also be used as a monetization tool via AdSense for Search, displaying Google Ads next to the search results.
To create customized search engines for your sites, just log in to your Webmaster Tools account, click on the site you want to add it to, go to the Labs section, and choose Custom Search. You will be guided step-by-step on what to do. Once your CSE is created, you can manage it directly from your Webmaster Tools account.
You can also create a custom search engine for your site the old way, via Google Custom Search.