By 
A spider is a software program that travels the Web (hence the name “spider”), locating and indexing websites for search engines. All the major search engines, such as Google and Yahoo!, use spiders to create and update their indexes. A spider is a program that visits websites and reads their pages and other information to create entries for a search engine’s index. Major web search engines have such a program, which is also known as a “crawler” or “bot.” Spiders are usually programmed to visit sites that their owners submitted as new or updated. There are many thousands of different types of spiders.
crawling
Crawling is fast, thanks to multithreading that allows up to 50 concurrent threads. Crawlers can also be used to automate maintenance tasks on a website, such as checking links or validating HTML code. Additionally, it can be used to collect specific types of information from web pages, such as collecting email addresses (typically for spam). Internet crawling can start from a single point (starting with a popular website that contains many links, or from indexes of older, existing websites). Crawler (also known as web bot or web spider) is a software program that can download web content (mainly web pages but also, in some cases, images, documents, and other files) and then follow links within these web pages to download the linked content.
Visual
Visual Web Spider extracts the title, keywords, description, plain text content, HTML content, size, and last modified time from web pages. It allows you to export the extracted data to Microsoft Access database, text file (ASCII), Excel file (CSV), HTML file, mySQL database, directory management system. Visual Web Spider is a website crawler, a fully automated multithreaded web robot. Helps you complete a database of websites, META data, and locate relevant information for research purposes.
Look for
Search engines such as Lycos and Alta Vista have become a commercial success as a result of our desire for information. Search Submit comes in two flavors, Basic and Pro, with the Pro version offering the advantage of giving you more control over how your listings are presented, helping to increase your click-through appeal.
Spider technology is necessary because the amount of information added to the Internet on a daily basis is more than any human team can index. Spider technology is not unusual; It is now used by all the major search engines. However, the advanced ways in which Google can compare the collected data is what has put them over the top.