Planning websites takes a lot of thinking about where content will live and how people are going to find it. It is easy to think of content organized in logical groupings, because that seems to be the way our brains organize things we know about. But visitors to your website don't know about all your content, so how do you make what you have obvious and easy to find for them? This is the job of an information architect and the whole purpose of navigation. Considering navigation and offering multiple methods is a key step in planning an effective website.
Index navigation is pretty old fashioned and most websites nowadays don't use it. The clearest example of this is the index at the back of a book. Items are listed in alphabetical order without special groupings. There are some clear benefits to this type of navigation. The biggest benefit is that there no learning curve at all. There is no risk that you may have confused or made it difficult for your visitors to find your products/content by placing them in the wrong category. Alphabetical organization is obvious and everyone understands how it works without even thinking about it. It's about as pre-cognitive as you can get a navigation system to be.
The draw back to index style navigation is when you have a lot of products/content, it makes for an overwhelming experience. Pages either become very text-dense and hard to read or the become very long and require much scrolling or paginating.
Either way, a site index normally has a pretty small foot print, can be tucked somewhere convenient and out of the way, and is handy for search engines. The ubiquitous "site map" is a variation of an index page, but normally these contain some sort of category grouping on-page also. This makes it a hybrid of an index page and a directory organization system. Sort of like a 2D version of your directory based website.
There are a million many different kinds of directory navigation. The traditional drop down list is very popular and something everyone knows. Directory navigation it the dominant navigation type of navigation on almost every website.
Directory style navigation is what we are thinking of when we group content logically. This is a very popular way to organize a website and almost every website has an information architecture based on this sort of logical model. For instance, if you have New York Yankee base ball caps on your website, you might create a top level category for Hats, then a second level category for Baseball Hats and list the hat there. Or you may create a top level category for Baseball Apparel, then a second level category for New York Yankees then list the hat there. There are many ways to create logical navigation to your products and/or content. Deciding what is the best grouping is difficult and depends on what you think your audience is looking for.
Hats » Baseball Hats » Yankees Baseball Hat
This concept of trying to understand how your audience thinks is very important. It may not be intuitive to your visitors if you organize your website based simply on how you understand your products and/or content. As a website "insider", you know your website's content very well. But all your visitors are "outsiders" and will need the content organized in a way that makes sense to them.So, with directory navigation, trying to understand who your target market is and what they are looking for is the best way to start. Thus is you know your target market for Yankees hats is already very familiar with baseball and is looking specifically for baseball apparell, you may create a top level category for American League, then a second level category for Yankees, then place the Yankees baseball hat there.
Search navigation is very popular and perhaps the most effective way to make content accessible to your visitors. The dominance of search engines over the past few years has transformed the way we imagine content retrieval. Instead of (sometimes) complex logical directory structures and extensive index pages, putting everything into one big bucket and using an automated tool to fetch things for us seems to work really well and more people are thinking in this interactive way than ever before when they are looking for things.
Additionally, search navigation tells us a lot about what visitors to your website are thinking and looking for. If you have Yankees Baseball Hats, but you are selling to an east coast market, should you call the Baseball Caps? Search navigation can tell you what your target market calls them. If you know 80% of you visitors are typing "Yankees Cap" into your on-site search box and are not finding your "Yankees Hat", then maybe you should rename the product to "Yankees Cap" so they can find it easier. This type of learning is invaluable for a website owner and is only really possible with interactive tools that have unrestricted input options, such as search navigation.
While search navigation seems to work well for the people involved (visitors and website owners), it does not work well for the machines involved, especially other search engines. Search engines such as Google and Yahoo learn about new content by following links. They do not type words into on-site search boxes and gather the results. If your homepage has search navigation as the only option, it will become a "barrier page" instead of a doorway to your site for spiders. Thus, your website content will be very accessible to visitors of your website, but it will not be accessible to everyone else in the world. This severely limits your ability to attract anyone who does not already know who you are. So go ahead and put a search box dead-center on your home page, just make sure you have links to content somewhere else on the page so the spiders can get in too.
Keyword Navigation (tagging)
Keyword navigation is relatively new to the world of navigation and comes directly out of the blog world. Keyword navigation could be considered a type of directory navigation, but there are some real differences that are big enough that I believe it needs to be considered on its own.
Keyword navigation is most often realized as a "tag cloud". The tag cloud is build and maintained by an algorithm based. There are various ways the algorithms may weight keywords. One simple example is by the amount of content contain those keywords. If you have 5 articles on the topic of "baseball hats" and 2 articles on the topic of "baseball bats", then the first topic will be seen as more popular and get a greater weighting. When the tag cloud is rendered on a webpage, the phrase "baseball hats" will normally be bigger and bolder than "baseball bats".
There are some really great things about keyword navigation and tag clouds. They are dynamic. They update automatically. This makes them a type of navigation that can learn and grow. Also, the rules they use can be changed which offers even more possibilities. They are link based. This makes them accessible to search spiders as well as humans.
There are some drawbacks to tag clouds also, but most of these have to do with the fact that this is a new method of navigation. A lot of people don't know about or understand what a tag cloud is. Also, tag clouds seems to work best on websites with lots of content that is updated often. Websites with a fairly static product offering could use a tag cloud navigation tool, but the dynamic nature of the tag cloud would not be utilized and it would just become a funky looking static directory-type navigation system. Also, tag clouds kinda look funny and are not intuitive for a larger part of the online audience.
As time goes on and tag clouds are refined, this may become a very strong navigational tool with much wider application than it enjoys now.
Promotional navigation is a very broad term for any sort of navigation that points to specific items and does not exist within a logical or automated structure. While that may seems like a catch-all, it sort of is. There are a lot of different kinds of promotional navigation and lots of ways to implement it on the internet.
Example 1: The homepage of your website has an ad for a 20% discount on all baseball hats and links to a page that describes the promotion in more detail.
Example 2: A blog post on a third-party clothing website has a review of the coolest fall styles and mentions Yankee baseball hats. They also link to your Yankee baseball hat as an example.
Both of these are different types of promotion navigation even though they are very different. Because there are so many possibilities, it may be helpful to create some sub-goups.Navigation created by the site owner
In summary, we see that there are multiple kinds of navigation for a website. Each has its strong points and weaknesses. Understanding your target market and being able to learn from them is important and requires flexibility. Once you choose why kinds of navigation to use and think it through, it is also important to plan how to implement changes based on visitor behavior which may not have been anticipated. If your directory structure is too complex, it may be very difficult to change if your visitors are getting lost a lot. Likewise, over-simplified navigation like index lists may present visitors with too much information, leading to high bounce rates. The ability to grow, adapt, combine and refine navigation based on visitor behavior and feedback, as well as internal website goals, is just as important as setting up the initial navigation schema. And it keeps web site management fun. :)