Taxonomy is the scientific practice of categorizing things.  For example, taxonomy is use to classify organisms.  The Dewey Decimal System was created in the 1800's and is a taxonomy.  The Library of Congress has a taxonomy.
When you get right down to it, a taxonomy is a preset tagging system.  In Dewey, if a book is on Technology it falls within "600".  In the Library of congress, the same volume would fall within "T".  These tags create:

• structure
• hierarchy
• standards

They also tend to be:

• specific to one use
• non-adaptive

Folk Taxonomy or Folxonomy allows users to create tags which means that you can have a virtually infinite set.  Systems that use folxonomy include Drupal, Joomla, Flickr, YouTube, Revver, and Google Video. 
Folxonomies are used when:

•  you don't know how many elements might need to be classified
• when you need a classification system that can deal with disparate elements.
• Folxonomy also creates structure, but it is much more organic than a taxonomy. 

These tags allow for cross referencing, aggregation, and indexing of ANYTHING and give search engines like Google something to latch onto.  It is the reason that photos and video can be indexed online.
The information, videos, and images on the Internet defy one taxonomy system so in comes the use of self tagging.  Tagging is important in order to be found in a vast sea of ones and zeros that gets bigger every day.  It is important to try and use some common tags and some unique tags.  This will give your offerings the most chance of being discovered.
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