Sometimes I want to have some means of structuring my tag cloud. For example, I tag a lot of researchers' home pages with their last names as tag. So I want to have all these last names together in one spot.
Furthermore, why do I have to say event every time when I tag something with conference, just so I can also retrieve it under the more general term?
Relations
To solve both problems, BibSonomy has introduced relations. A relation between two tags signifies that one of the tags is regarded as a generalization of the other.
When I tag something with conference, and I have the relation conference->event stored in BibSonomy, I say that I also consider that resource I tagged to be concerned with event in some sense.
Relations can be entered just in the way shown above: just write an arrow "->" or "<-" between two tags. The tag pointed to will be the more general one. This can also be chained: you can tag, say, a page related to pet cats with cats->pets->animal.
We call those tags that are more general than another tag, i.e., those at the pointy end of an arrow, concepts.
Using Relations
Now how can relations be used? First, they appear in your tag box on the right hand side. Note the upward and downward arrows next to all tags that are concepts. Clicking on an upward arrow shows the relations of that concept, clicking the downward arrow hides them.
Second, you can use concepts to retrieve posts. Note that when on a tag page, such as http://www.bibsonomy.org/user/schmitz/linux, you will be offered the possibility of seeing the tag linux as a concept. That way, you will also see those resources not tagged with linux themselves, but with a direct subtag thereof, for example ubuntu (assuming I have the relation ubuntu->linux in my relations).
Editing Relations
Entering relations can be done on the fly, with the arrow notation shown above. They can be browsed under the myRelations link. If you want to edit your relations, hit the edit tags link in the top right corner. You will be offered the option of adding and removing relations. Furthermore, note that when renaming a tag (say, you've made a typo and spelled ubutnu instead of ubuntu), there is an option on the edit tags page to update relations containing that tag, too.
Have fun,
Christoph