You can associate tags to each of your entries from the entry edition interface (details). These key-words will then be listed in your blog's menu, and your visitors will be able to sort your entries by tag.

You can also add tags from the entry list screen by selecting entries and choosing the Add tags action. You will then be presented a field where you can enter tags separated by commas. Click on Save to add your tags.

From the left sidebar menu, click on the Tags link to display the alphabetical list of the tags you use, followed by the number of entries with this tag.

Click on a key-word to display a list of the related entries in the lower section, and a tag edition field in the upper section. The modification will be applied to all the instances of the tag.

The entry list of a tag also allows you to apply certain actions on entries (publish, unpublish, delete, etc.)