How to use Taxonomy to tag/categories content?
The steps to follow to be able to use a vocabulary with nodes of a content type are two:


Create the vocabulary, if it doesn't exist, on admin/structure/taxonomy.


Associate the vocabulary with the content type on; for example, for the Article content type, the page is admin/structure/types/manage/article/fields.


Add a new field, using "Term reference" as field type.





Click the "Save" button, and select the vocabulary that is used for the taxonomy terms that will be added.





Change the settings in the page that appears after you click on the "Save field settings" button.



This is different from Drupal 6, where the form to create (or edit) a vocabulary also allowed to select which content type should use that vocabulary. In Drupal 7 is changed, as a vocabulary can be associated to other entities, such as users (see admin/config/people/accounts/fields) and vocabularies (see admin/structure/taxonomy/forums/fields for the "Forums" vocabulary).