A SEO spider is a program which browses the World Wide Web in a methodical, automatic manner.
This process is called spidering. Many sites, in particular search engines, use spidering as a means of providing up to date data. The SEO spider creates a copy of all visited web pages in a huge database and then searched when a user enters a keyword into a search engine.
A spider will start with a list of URLs to visit, index and as it visits these websites it comes across other links which in time will return to visit those websites as well. This process can sometimes take weeks even months.
Spiders like links
This is why it is important to have some incoming links from other websites. Spiders can follow these links and find your website, it will then index and enter you into their database for future searches at search engines.
Some websites can not be indexed by spiders due to the way they were designed. This is where a SEO Sitemap has to be used to insure your whole website is indexed.
The spider in most cases will index every incoming link, outgoing link, internal links, web titles, headers, meta tags content and so on. In the SEO Keywords section you will learn where to place keywords. This will tell the spiders what your website is about and at the same time attract human visitors.
Helping the SEO Spider
Sitemaps can overcome most of the above problems and trying to maintain a site structure that is no deeper than 4 tiers can hugely benefit rankings.