A search engine is a software system that is designed to carry out web searches. They search the World Wide Web in a systematic way for particular information specified in a textual web search query. The search results are generally presented in a line of results, often referred to as search engine results pages (SERPs). The information may be a mix of links to web pages, images, videos, infographics, articles, research papers, and other types of files. Some search engines also mine data available in databases or open directories.
The essense of SEO is all about search engines and how, when, why and if a website appears in them. It is important part of the SEO to check your webpages how they appear in most of the search engines out there. Some of them, like google and bing, have a dedicated webmaster tools for Search Engine Optimization.