Establishing interesting and useful relationships within the content on a website is a powerful method of retaining visitors and bringing them back. The Tag add-on is an intuitive relationship tool. It lets you freely apply relevant tags to content on your site. You can then display in your templates the relationships that emerge within this data to improve your visitors' experience. Relationships develop over time, naturally, intuitively. Tag brings this natural progression to your website.
Tag allows your users to view lists of entries that contain specific tag(s), or while viewing a specific entry, see other entries that are related by use of the same tag(s). These related entries can be ranked by most tag matches. Users can also subscribe to tags and view lists of entries that contain the tag(s) they've subscribed to. You can also display a list or a weighted "cloud" of tags that exist for your site.
Tags can be created/selected in the Publish page of the CP while creating or editing an entry. They can also be added to entries via Channel Form, or seperately through the Tag form template tag. You can have as many Tag fields (and groups) as you wish, as it is a field type.
If you already have a tagging system where tags are placed into a custom field, or if you've manipulated the Tag "feel" by assigning your entries to EE categories, Tag can easily transition your website. Using the built in Harvesting utility, you can easily harvest this data, and it will automatically convert all into Tag tags and apply them to your entries.