An e-commerce site's category pages are like a friendly virtual salesperson: These pages help shoppers find what they want by presenting a well-organized grouping of products.

A good category page will offer not just a number of products, but also a number of ways to sort them (price, style, specs). The best category pages actually help clinch the deal by offering such a sensible grouping that they almost read the customer's mind.

These pages aren't easy to design. Throughout the e-commerce industry, category pages are some of "the worst" in terms of effective design, says Jakob Nielsen, a leading Web usability guru.

"They're very poor in helping people understand the product line or the selections within that particular category."

Often, category pages are designed as if someone said, "Let's just dump all the choice here." Notes Nielsen: "Okay, here are ten choices, or fifty choices, or a hundred choices" - but, he asks, "What are the differences? - That's almost never described in any kind of meaningful way."

"So you're typically just left with saying, 'Okay, I've got to click on all of them.' And if you have ten (choices), maybe they'll do it, but that's already a drag - and if you have a hundred, there's no way."

An upper level category page at Toolking.com clearly groups many items into different categories.

Don't Overwhelm Me
The well-designed category page never offers a series of links that are undifferentiated; links must be descriptive and organized into obvious categories.

There's a good reason that some e-tailers don't properly group and organize their products: it's hard to do. "You've go to think about each category on its own - what are the criteria for each one?" Nielsen says.

But this homework is well worth it. A properly organized category page "is really how you can pull in the customers, by helping them narrow down their choices."

"You can ask any salesperson: If you want someone to buy, you don't give them fifty choices, because they're not going to be making up their minds - you give them two or three choices."

This narrowing of the choices for the purpose of sealing the deal is a timeless retail rule, one that far pre-dates e-commerce. But, Nielsen says, "The sad thing is we've forgotten about those thousand-year well-learned lessons."

Tell Me Where I'm Going
A category page that's properly designed to do its main job well - allowing shoppers to quickly sort through products - must offer information in brief tidbits.

"Each product must be described in enough detail that you can get a predictive view, to know before you click, approximately what it is," Nielsen says.

The home page of Perfume.com embeds a category section, allowing users to dive right into the site's large selection.

This usually involves having a small photograph, though not always. However, "Always it's the price," he says. "And then, it's usually the brand of the vendor, then sometimes it's some amount of specs - but just a few of the key ones."

A poorly designed category page simply presents a long laundry list of items with no accompanying information. If a category page says, "'Okay, here's the name of the cheese, and you know there are a thousand cheeses, it doesn't help you at all, unless you know exactly what you want, and then you're going to use a search engine anyway and go straight to it."

The importance of making links descriptive, "is a classic - I think it was the number one Web design guideline from '91," Nielsen says with a laugh. Yet some sites still say 'click here.' "It does continue to be problem - there's a lot of non-descriptive links where you can't predict what you're getting."

Can't We Just Use the Search Tool?
A site's search tool is a kind of ultimate category page. It sorts products into a category called "results" based on a shopper's keyword.

But search has its limitations. Search is useful "only if you know what you want," Nielsen says. "The problem is that, quite often, you don't know exactly what you want. So, okay, you might search for a five megapixel camera, but on the other hand, maybe you'd be quite satisfied with a 4.9 megapixel camera that has some other things you want, or is half the price."

In other words, "Search is quite often too specific," he says. Instead of one exact type of cheese, the shopper might say: "I need to get some interesting new cheeses to serve for a party; something that not everyone has had before."

This is when the e-tailer can really lead the customer: By laying everything out in a logical grid, a good category page helps a befuddled shopper find their way.

Your Home Page is Really a Category Page
Although category pages are typically a site's mid-level pages, the home page is actually a kind of category page. It's the first place that shoppers stop and ask: What's my next step? Where do I click to find what I want?

A home page should, of course, clearly display what merchandise the site sells. But more importantly, the home page should enable users to start shopping immediately - visitors shouldn't have to dig far to begin shopping.

To encourage shopping on the home page, the page's navigation panel should allow visitors to dig right into your product categories. Your home page shouldn't just provide a link to the catalog; it should be the front page of the catalog - see links, start shopping.

The key point: show as much product hierarchy as possible right on the home page.

Nielsen cautions against over-emphasizing promotional items on the home page. Displaying sale items too prominently can affect a shopper's perception of a site's merchandise. For instance, a site that sells high-quality jewelry, but has a large "Sale!" display of cheap pieces on its home page, may lead users to think the site sells lower quality goods.

Classify It - But How?
A category page performs a minor magic trick: it classifies a site's products clearly, in ways that makes sense to the mythical "average shopper."

This isn't an easy task. "Classification is hard to do," Nielsen writes in his research. "And it's especially tricky when the people charged with creating the classification schema for the Web site are much more familiar with the product or service than are the site's customers."

Nordstrom's winnowing tool gives shopper enormous control over browsing. It enables them to choose Category, and then define that pick with various sub-choices.

He recommends offering multiple classification schemes - different customers think differently about the same item. So a homemade coffee cake can be grouped under (at least) three different categories: "desert items," "gift items," and "holiday specials." On the other hand, he notes that some small sites really don't need much advanced classification - all the products can be displayed on one page without getting too fancy about it.

In any case, it's important to be consistent about your classification scheme. If your site's search tool shows a dozen portable music players, your category page for portable music players should also show all twelve. (Nielsen's research showed that not all sites do this).

Advanced Technique: Allowing Customers to Do Their Own Categorization
If feasible, it's best to limit the number of category pages you use to show products in a given category. Some sites offer a list of products that's hundreds of items long, sprawling across ten or more pages. In real life, users generally won't click past the first two to three pages.

If your site's inventory is large enough to require a dozen category pages for a single category, Nielsen recommends using a technique called winnowing.

In Nielsen's focus groups, 27 percent of the "sales catastrophes" (failure to make a sale) were caused by a user's inability to find a desired item - even though that item was somewhere on the site.

A winnowing tool can help prevent these failures. Winnowing tools allow shoppers to take control of the product sorting, enabling them to peruse products based on their own choices. An ideal winnowing tool allows users to select three or more criteria at the same time, and then click Submit - quickly narrowing the choices to exactly what they want.

When adding a winnowing tool to category pages, Nielsen cautions merchants to make sure that the winnowing criteria - size, color, style - are very clearly understood by shoppers. Price is often a good choice to include, and the other choices should be so obvious as to be inarguable.

A properly designed winnowing tool "separate great sites from good ones," he notes.

James Maguire is a contributor to ECommerce-Guide.com. His weekly feature appears every Monday.