CMS is a particle detector that is designed to see a wide range of particles and phenomena produced in high-energy collisions in the LHC. Like a cylindrical onion, different layers of detectors measure the different particles, and use this key data to build up a picture of events at the heart of the collision. Many of CMS Software components (CMSSW) are hosted on Github. You can find here a few FAQ and tutorials on how develop in CMSSW.