Search Engine
-->Search engine is also a type of website where an internet user could search and find specific topic. Search Engine are well scripted. Using their crawler called spider, it crawls all web pages and add it to their indexed pages.
Though all website which you can search for a topic could be considered a search engine, but the big difference here is that search engines like Google, MSN and Yahoo have no human intervention when crawling a website. Unlike directories where you could also find relevant topic, before those submitted sites could be added in their indexed pages, the owner should approve it first.
Google, as the number one search engine at present has $200 billion plus capital running on it (correct it if I am wrong). Because it this, Google implemented further logarithmic processes where most of the small scale online entrepreneurs have been affected. The most affected websites are web directories.