One of the key elements of Web 2.0 is the concept of tagging, giving each item in your database an arbitrary number of keyword tags and using those tags to structure the user's interface to the data. Allowing users to tag items allows for a richer categorization of the data, and doesn't restrict items to just a single category. From the perspective of a user browsing the data, tags allow for a quick way to find related items quickly.
From Rails, tagging is managed with a plugin named Acts As Taggable On Steroids, or AATOS. (Why the name? Because it's a replacement for a previously created Acts As Taggable plugin — apparently you need to inject illegal substances to improve your tag plugins these days.) Anyway, the AATOS plugin provides support for adding tags to an arbitrary ActiveRecord model in your project.
To start using AATOS, you need to first install the plugin. Most Rails plugins are distributed via a Subversion server, which you can load into your project, or if you are also using Subversion to manage your project, you can add the plugin site as an external site (in the same way that Edge Rails is added as an external site). The standard Rails mechanism for installing a plugin is the provided plugin install script. Installing AATOS via this command looks like this:
$ ruby script/plugin install -x http://svn.viney.net.nz/things/rails/plugins/acts_as_taggable_on_steroids
The URL is the address of the Subversion ...