In Drupal, tagging your content with predefined keywords is an excellent way to group or organize you content to be retrieved by others. For example if the content you create is specific to people in certain profession, you can create a category called "Professions" and list the various professions you cater to. As you create your content you can tag it with the most appropriate profession (s) the content may be applicable to. You can create categories for certain content types or across all content types from Administer » Content Management » Add vocabulary.
Once you create a vocabulary you can list various terms and enable hierarchical tagging, multiple keyword tagging, or free tagging. Once your content is tagged you can use URL aliases to create friendly names for each of your taxonomy terms from Administer » Site Building » URL aliases.
In this video you will see learn how to enable vocabulary and tag an image content to be retrieved in different ways.