This works by linking attributes to words that are meaningless per se, for the computer. Done this, you can connect to other items sharing the same attributes. (quick and dirty basic youtube introduction video)
Having given meaning to words, computers can be much more useful, since they can understand the context.
Here is a nice little graphical ontology viewer with limited but good data set.
(click on the nodes!)
(on the adress bar, you can directly enter a person name. The n=5 value describes how many child nodes divert from the search term)
AFAIK, the technicals problems are:
1) extract metadata from non-structured or semi-structured content automatically
2) integrating content with metadata, linking different standards of semantic nets
3) real life applications
However, thanks to automatic metadata extraction, the semantic net makes advances.
This clickable map of semantic networks, each node being a big semantic databases of different subjects and their interdependancies shows the enormous potential:
What a pity, that apparently we cannot really use it for now
