In reply to Giorgos Sarigiannidis.

Tags are a different concept than categories.

See them as keywords (which is its German translation, too), so kind of hints, shorthand notices, not as a major way to categorize content.

The tagging concept also only starts to work out well if you have a bigger amount of data. Just take a look at social bookmarking sites (like Delicious or Scuttle), location directories (like Yelp) or Wikis (like DokuWiki). Oh, and of corpse search engines :)

cu, w0lf.

ps: I see the concept working out very well with my scuttle install, which mainly acts as my personal cross-platform bookmark service, and has been in use for over 6 years ;)