Chef instructions are recipes, which are bundled together in cookbooks. A cookbook contains at least one recipe, which performs some actions. Cookbooks can contain multiple recipes and other resources such as template and files.
At its most basic level, a cookbook is a folder (named as the name of the cookbook) containing at least a recipes folder which contains at least a default recipe file,
default.rb. Files are typically stored in a files folder within the cookbook folder, and template files within the templates folder.
While both, Puppet and Chef use Ruby, Puppet is a Domain Specific Language which makes it look and feel like its own language, whereas Chef is structured more like Ruby itself.