Put simply, Puppet is a system for automating system administration tasks. To learn more, read our big picture overview of Puppet, or take a deeper look at what Puppet can do with the Puppet Introduction. There's also an about Puppet page which gives the highlights of Puppet's functionality.
The FHS defines the /usr/local as the “tertiary hierarchy for local data installed by the system administrator.” Translated into English, this means put apps you build yourself here. Think of /usr/local as a “safe haven” for command-line, open sou