What are Spiders, Robots and Crawlers and what are their functions?

They are all the same and are different names of a search engine automated program that is responsible to read through webpage source and provide information to search engines. They are helpful to crawl and index webpages in search engines.

All three words have same term or definition because all are same. They access your web page when a query is occurring on the search engine and show the result on SERP.

Spider - The browsers are like a program and to download the web page.
Crawler – The program is automatically to follow the links are web page..
Robots - It had automated computer program can visit websites. It will guided by search engine algorithms It can combine the tasks of crawler & spider helpful of the indexing the web pages and through the search engines.