jq is a command-line JSON processor.
If you want to learn to use jq, read the documentation at http://stedolan.github.com/jq. This documentation is generated from the docs/ folder of this repository.
If you want to hack on jq, feel free, but be warned that its internals are not well-documented at the moment. Bring a hard hat and a shovel.
You can find some basic build instructions at http://stedolan.github.com/jq/download