You'll use Puppet's declarative language to describe the desired state of your system.
You'll describe the desired state of your system in files called manifests. Manifests describe how your network and operating system resources, such as files, packages, and services, should be configured. Puppet then compiles those manifests into catalogs, and applies each catalog to its corresponding node to ensure the node is configured correctly, across your infrastructure.
Several parts of the Puppet language depend on evaluation order. For example, variables must be set before they are referenced. Throughout the language reference, we call out areas where the order of statements matters.
If you are new to Puppet, start with the Puppet language overview.