Google.com is a type of website known as a search engine. Just like other websites, it's made up of individual webpages. Generally, the webpages you'll find on Google are called “search engine results pages” (SERPS) which contain lists of webpages that best match a given search query. Google.com uses bots known as “spiders” to “crawl” (ie, scan) webpages across the World Wide Web. These bots identify what kind of content is on each webpage it comes across, and “index” it. Each indexed webpage is ranked according to quality, usefulness, and some other factors. That's why you'll (usually) see the most relevant webpages for your search engine query at the top of the SERP.