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Rather than defining all parts of the build in a single file (like make or cmake), bitbake separates each component that needs to be built into its own file. This file is that component's recipe. Each recipe can either provide one or more packages (for example, libraries will often provide a lib and lib-dev package), or modify an existing recipe. The latter is useful when the package you want to modify is not under your control. Whether a file represents a new recipe or changes to an existing one is controlled by its file extension: files for new recipes end in .bb while those modifying existing recipes end in .bbappend.
Check the Recipes page for more information.
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