How did this all start?
It gained traction when people realized that a lot of places on the desktop refer to the same things, but they all do it incompatibly. (me, me, me, me, me and I might be in your IRC, mail program and feed reader, too.) So they set out to change it. Unfortunately, such a change would have required changes to almost all applications. And that is hard. An easier approach is to just index all files and collect the information from them without touchig the applications. And thus, Tracker and Beagle were born and competed on doing just that.
However, indexing has lots of problems. Not only do you need to support all those ever-changing file formats, you also need to do the indexing. And that takes lots of CPU and IO and is duplicated work and storage. So the idea was born to instead write plugins that grab the information from the applications while they are running.
But still, people weren’t convinced, as the only things they got from this is search tools, even if they automatically update. And their data is still duplicated.
What’s a sematic desktop anyway?
Well, it’s actually quite smple. It’s just a bunch of statements in the form <subject> <predicate> <object> (called triples), like “Evolution sends emails”.