It has its own folder in ~/Library, and no less than 7 of its own Private Frameworks. But what is it, and what does it do? This is as much as I know about it.
Core Spotlight
Indexing and search diagrammed, leading to the diagnosis of problems with Spotlight, and their resolution.
Apple’s Mail, Notes ad other apps now use Core Spotlight, which protects the privacy of their data, but stops search apps from doing their job. And worse.
The Eclectic Light Company 