Baloo is the file indexing and file search framework for KDE.
Baloo focuses on providing a very small memory footprint along with with extremely fast searching. It internally uses a mixture of sqlite along with Xapian to store the file index.
Like its predecessor, Nepomuk, Baloo is not an application, but a component which can be used by developers within applications. Applications like the Kontact Suite, Dolphin and Gwenview, as well as the Plasma Desktop itself, use the Baloo framework to provide search results.
The Baloo developers maintain a list of currently active tasks (requires login). Additionally there is also the bug tracker.
As of KDE version 4.13, Baloo has replaced Nepomuk.
The Nepomuk project in KDE aimed at implementing the vision of "The Semantic Desktop" using RDF. After years of trying the developers felt that while many of the ideas behind "The Semantic Desktop" were great. Implementing them using RDF and classical semantics was not the way to go.
The Baloo project is just a file indexing solution for KDE. Everything that Nepomuk provided is now handled by other software which are part of the KDE Umbrella.