A web site whose content is accessible by search engines so the content can be indexed.
A crawler follows the links on the web. A crawler is also called a robot, a bot, or a spider. It goes around the internet 24/7. Once it comes to a website, it saves the HTML version of a page in a gigantic database, called the index. This index is updated every time the crawler comes around your website and finds a new or revised version of it.
Alternative "non-crawable" website. No indexing occurs.