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Displaying hierarchical content is a common request of database-driven web sites. Drupal gives you great flexibility in doing this without writing a single line of code, using a couple of battle-tested modules. As an example, let's say you're building an automotive web site. Your site is going to have a list of automotive brands and the vehicles they build. You'd like to only have to enter each brand's information into the database once, then simply refer to that brand whenever you add one of their vehicles to the site. In this example, I have 2 brands, Chevy and Jeep, and each brand has 3 vehicles.

Drupal's powerful CCK module allows you to define different content types for your web site. The Node Reference module, included with the rest of CCK, allows you to set up hierarchical relationships between content types.

I went ahead and set up 2 simple content types:

The "vehicle brand" content type is just a simple content type with the body field removed and no additional fields added. The user just needs to enter in the brand's name.

The vehicle content type is similar to the vehicle brand content type in that there is no body field, but I did add a Node Reference field that allows the user to select the brand that the vehicle belongs to. I set it up as a select list with the following settings:

Once the content types are set up, you need to input some sample data. In my case, I entered the brands and vehicles as shown above.

The trick now is to figure out how to display the brands and vehicles in a hierarchical list of links that will bring us to each of the nodes that we just created. This can be accomplished using Views Relationships.

I went ahead and set up a basic node view named "vehicles", with some basic settings:

Row Style: Fields
Fields: Node Title (no label, linked to its node)
Sort Criteria: Node: Title (ascending)
Filters: Node: Type = Vehicle, Node: Published = Yes

These simple settings will get us an alphabetical list of all vehicles. This is a good start, and you'll be able to build on this to get what you want. To achieve this, you need to add the relationship between the vehicles and the brands:

In our simple example, there is only a single "content" relationship available, the node reference relationship we set up earlier. Be sure to require the relationship and name it "Brand Relationship" on the settings page after you select it.

Now, you're going to want to add the Brand's name as a field so that it can be used to group the vehicles in the output. The important bit here is that we want to select the "title" field for the "vehicle brand" and not the "Content: Brand" node reference field for the "vehicle" - a subtle but important distinction. When selecting the "Node: Title" field, you'll have the option to use a relationship - you'll need to use the "Brand Relationship". You'll also want to change the label (for now) to "Brand" and check the "Exclude from display" and "Link this field to its node" checkboxes.

The "Exclude from display" option pulls in the brand titles, but doesn't display them - this will be handled by the grouping that you set up in a minute.

At this point you should see the alphabetical list of vehicles with no brand information visible.

In order to view the data as a hierarchy (and see the brands), we need to group the fields by the brand name. Do this by clicking the settings button for the "Style" in the "Basic Settings" section.

Select the "Brand" as the field to group by. If we hadn't changed the label of the brand's "Title" earlier, then we'd have to choose between two options that are both named "Node: Title" - the vehicle title and the brand title. That's confusing.

That's it, you're done! Take a look at the view's preview and you'll see the desired results. Feel free to go back into the "Node: Title" for the brand and remove the "Brand" label (remember, it was only there to help us choose the correct grouping field) - doing this leads to the view result looking like:

It is important to note that it is also possible to do all this by setting up the vehicle brands as a taxonomy, and not a separate content type. It makes sense to use taxonomy if you aren't going to need to fully describe the vehicle brands. In this example, it is assumed that the vehicle brand content type is going to have additional fields describing the brand (Nationality, HQ location, number of employees, logo, etc...)

There are a lot of ways to further customize the look of the resulting hierarchy with theming, but this should get you moving in right direction.

UPDATE: I went ahead and exported the 2 content types and the view and provided below for you to download and import.