The crawler or Spider or a Bot refers to a special computer automated system which is generally used by the search engines for indexing the relevant sites. The crawler crawls over the different kinds of web pages and copies the content and stores them. When a user looks for a specific keyword then the crawler looks into the storage and provides the search engine with the relevant information and that too without taking too much time. The crawler mostly goes through the text of the site only and is able to capture that and store it in its own database. It also crawls over the website that it has covered in the past and checks it for new developments and changes and then acknowledges the same. It is very useful for the search engines and for SEO purpose as it makes the search engine’s work faster and more efficient.
Apart from the search engine crawlers there are others kinds of BOT or crawlers also which may work on an altogether different set of instruction. For instance, the crawlers may be used someone to simply get a list of email addresses which are then used for spamming. Until and unless the website is not crawled it may not show up in the search engines result and this is why it is considered to be very important.