This article is the first article within the sequences of articles/tutorials about Magento Layered Navigation.

What is Magento Layered Navigation?

Magento Layered Navigation is a piece of functionality that allows customers to filter categories in the following way:
If you have to example a category named "Apparel" and the products inside this category have
attributes such as color, manufacturer, etc., you can tell Magento to display a sidebar that will allow the customers to filter the category named "Apparel" by those attributes (and some others - will be explained later in this article). The next image shows an example of such sidebar:

How to make such a sidebar appear on my desired category?

To make such a sidebar appear on a desired category and contain a desired attribute, you must follow next 3 simple steps:

1) Make the desired category "anchor": Go to the admin panel -> Catalog -> Manage Categories. On the left, click on the category that you want to make filterable. On the right, click on the "Display Settings" tab, and choose "Yes" on "In Anchor" field.

2) Add filterable attributes: Go  to the admin panel ->Catalog -> Attributes -> Manage Attributes.  Add attribute which you wish to filter by (Detailed explanation about adding/managing  attributes -here). Make sure that you choose "Filterable (without results)" or" Filterabe (with results)" in"Use In Layered Navigation" field (To do so, you must choose "Dropdown" or "Multiple Select" or "Price"  in "Catalog Input Type for Store Owner" field).  Also don't forget to add some options (on Manage Label/Options tab). Finaly, make sure that you added those attributes to some attribute set (Detailed explanation about adding/managing  attribute sets - in next articles), remember the name of this set for next section.

3)Apply attributes on some products: if in the previous section, you set "Filterable (without results)" for the attribute, you will not see this attribute on the sidebar, until you will have some products that has a value for this attribute. So, create new product (choose tha attribute set that you chose in a previous section), or update existing product (that has this attribute on it's attribute set), so that the field of the attribute(s) you added, will contain some value. Don't forget to set a desired category as a category of this product.

Now, on your desired category page, you should see a sidebar, that contains the attribute you added.

(Note: those 3 steps are 100% effective for clean Magento installation. If some modifications where done to the code/design files, those steps may not suffice.)

In the next article we will learn about the architecture of a Magento Layered Navigation.