Nepomuk is the social semantic desktop framework that was largely developed in KDE SC 4 and uses RDF (Resource Description Framework) data storage. The reported cost of developing Nepomuk was reportedly 17 million Euros, after the European Union had invested most of that money into its development for advancing the semantic desktop (update: There's some confusion over the Nepomuk investment and apparently not for the KDE portion). KDE Nepomuk finally reached a mature state in KDE SC 4.11.
Phoronix reader Eric Griffith pointed out an interesting mailing list post this weekend that was originally written back in December. The post is by a Nepomuk developer, Vishesh Handa, about Nepomuk's future. The most interesting bit from the mailing list post was "After a month of designing the solution and a month of implementing it, Baloo is working way better than Nepomuk does. So, I'd like to switch to Baloo by default in 4.13, while keeping Nepomuk in maintenance mode for more conservative distributions."
Baloo is a meta-data and search framework for KDE that prides itself on having a small memory footprint and being able to conduct fast searches. Baloo isn't a complete rewrite of Nepomuk as parts were derived from it and its research. Handa went on to write, "Baloo was also discussed in PIM Sprint and the PIM developers are happy to completely drop Nepomuk support for 4.13 and move to Baloo. Similarly, the telepathy developers are also working on moving KPeople away from Nepomuk."
For easing the end-user move from Nepomuk to Baloo, there will be a migration path for the captured data, but it seems for KDE SC 4.13 this year Nepomuk is on its way out the door.