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Where does it explain what Click is? No explanation on the front page, and first FAQ is "Is Click experimental software?"

Click is a C++ framework for constructing routers and IP middleboxes out of modular components, somewhat like a modular synthesizer for packets.

It's a pretty great codebase.

How specific is it to routing network traffic? Can it be used for more general data processing?

It's specific to processing IP packets, but not to routing. It has a really elegant interface for handling packet events.