Google Webmaster Guidelines > Hierarchy and Links
A clear hierarchy lets people and search engines know what is important in your site. Well structured websites create easier navigation for their users and present search engines a clearer view of what the site is about.
If you are making a website about different types of pets, it makes good sense if your home page was about pets.
If you made a website about pets but had a home page about turtles, then visitors and search engines might assume your website is more about turtles than it is about pets. Even if overall your site is about pets. This might seem obvious, but it is important to remember that no matter how good your dog page is, people won't look for dog info on a turtle site and search engines won't value it as much either.
This example speaks of the homepage but this theory is important to continue throughout your website - "poodles" and "golden retrievers" should be a subcategory of "dogs" not "cats" and so on.
Use text instead of images to ensure that those who can not see (whether they be people or search engine crawlers) can navigate your site.
Text links are important for webmasters to use rather than image links because people who are sight-impaired are unable to see what your images are. Image links are a problem for search engine spiders, who also can't "see" images. A website that has a text link navigation menu will be more easily understood and more likely to be indexed correctly by search engines than a site with image links. If images are an important part of your website, you will need to be extra careful to provide text alternatives to ease navigation of your site. The more accessible your website is to the sight impaired, the more accessible your site is to search engine crawlers. Text links and their importance to your website is covered more in depth in another guideline - "Try to use text instead of images to display important names, content, or links".
Having each page of your website reachable by at least one static text link is very important if you want search engines like Google to know about that page.
A Flash homepage might have links to the other pages of your website. Even though people looking at that homepage can see text and links and choose to follow them, search engine crawlers may not. If you want search engine crawlers to know about each of your pages so it can index them you must have, as they say in this guideline - at least one static text link to each page. Even though Google can read Flash and other technologies, they still recommend using good old fashioned static text links.
To make your pages visible to Google if you have for example a Flash home page would be to have beneath your Flash presentation some text links to your other pages that your Flash page mentions. In doing so, you are abiding by this guideline.
The reason it is recommended that you have at least one static link to each page of your website is so that you can be assured that blind users can navigate your website and search engine crawlers can crawl each page and therefore each page can be indexed.
A clear hierarchy allows your users to find the information with more ease. It also presents search engines a clear view of what is important to your site.
Using static text links ensures that both people who are blind and search engine crawlers can navigate your site.
A good way to determine if your site is organized well is to make a site map page or review your current site map page. Since a site map page is like an outline of your websites pages, it gives a good organizational view of your site. If that outline is clear and makes sense you probably have a clear hierarchy. Most small sites have no problem with this.
The way Google recommends to discern how a search engine spider may see your site is to view your page in a text browser.
A quick check to see what links are visible to a search engine spider can be done through a search engine spider simulator. There are many available on the web, or use our spider simulator.
From Google help pages:
From other sources:
This article was written by Patrick Sexton, who has been helping webmasters with the Google Webmaster Guidelines and other issues since 2005.