You can associate tags to each of your entries from an entry's editing interface. These key words will then by listed in your blog's menu, allowing you to sort displayed entries according to the tags of your choice.
You can also add tags from your Entries interface by selecting the relevant entries and choosing the add tags action. You will then be able to add a list of tags separated by commas within the given field. Click Save and your tags will be added.
Click the Tags link in the left-hand administration menu to view an alphabetical list of the keywords used, followed by the number of entries referring to it.
One click on a keyword will display the list of entries concerned, and give you the possibility to rename the tag within a field located at the top of the page. The modification will apply to all occurrences of the keyword.
The list of a tag's entries also allows you to perform some actions on those entries (publish, unpublish, delete, etc…).