What exactly is a spider? Electronic spiders, also known as bots or crawlers, are how the search engines find your site. These spiders are sent out on a mission to find information on the millions of websites that exist. They crawl through the Web making lists of words found on websites. These words are the key to people finding the information for which they search.
The spiders will begin crawling through the most popular of websites. They will then begin building their list of keywords and will extend their search by following every link on each site. If there is a link to your site anywhere on the Web, the spiders will find it.
Each search engine will have its own rules and standards but the process is basically the same. The spiders, however, can only read textual content and cannot click buttons or use drop down menus. They have limited power and capabilities.
Once the spiders retrieve the information, it is collected in a database which is how the search engines determine your ranking. This information is what people find when they use keywords to search for websites.
So with this in mind, you want your site to be spider-friendly so they visit you often. There are things you can do to invite the spiders to visit your site.
Spiders Like:
1. Robots.txt File
2. Head Section
3. Website Content
4. Site Map
5. Backlinks
Spiders Do Not Like:
1. Javascript Navigation
2. Doorway Pages
3. Frames
4. Images
5. Flash
Thanks to All
Source(seo-news.com)