Major Component of how search engines work is a robot, or spider, which is software that slurps up your site’s text and brings it back to be analyzed by a powerful central “engine.” This activity is referred to as crawling or spidering.
Think of a search engine robot as an explorer ant, leaving the colony with one thought on its mind: Find food. In this case, the “food” is HTML text, preferably lots of it, and to find it, the ant needs to travel along easy, obstacle-free paths: HTML links.
Following these paths, the ant (search engine robot), with insect-like single-mindedness, carries the food (text) back to its colony and stores it in its anthill (search engine database). Thousands and thousands of the little guys are exploring and gathering simultaneously all over the Internet.

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