Both are same, spiders and crawlers both are used to scan the webpages and after that it index that information in Google Database. Google Bot is also it's another name.
Spider is a program which on a certain time go and read a webpage like a user ( it means it is something like a browser).
Crawler is the manager of spider.it read the links and says to spider when, where and how deep goes into a website.
Spiders and crawlers are the same with different names. They are search engine automated programs that are responsible to read through webpages and provide information to search engines.
Spider- A spider is a program run by a search engine to build a summary of a website’s content (content index). Spiders create a text-based summary of content and an address (URL) for each web page.
Crawlers - Crawlers or Crawling takes place when there is a successful fetching of unique URIs which can be traced from valid links from other web pages.
Crawler is basically named by Google that it's crawler visits websites to index pages. Otherwise crawler of other search engines such as Bing, Ask, etc. are called spiders.
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Crawler is the manager of spider.it read the links and says to spider when, where and how deep goes into a website.
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Crawlers - Crawlers or Crawling takes place when there is a successful fetching of unique URIs which can be traced from valid links from other web pages.