Wikipedia
BitBake is a make-like build tool with the special focus of distributions and packages for embedded Linux cross compilation although it is not limited to that. It is inspired by Portage, which is the package management system used by the Gentoo Linux distribution. BitBake existed for some time in the OpenEmbedded project until it was separated out into a standalone, maintained, distribution-independent tool. BitBake is co-maintained by the Yocto Project and the OpenEmbedded project.
BitBake recipes specify how a particular package is built. It includes all the package dependencies, source code locations, configuration, compilation, build, install and remove instructions. It also stores the metadata for the package in standard variables.
The BitBake recipes consist of the source URL (http, https, ftp, cvs, svn, git, local file system) of the package, dependencies and compile or install options. During the build process they are used to track dependencies, performing native or cross-compilation of the package and package it so that it is suitable for installation on the local or a target device. It is also possible to create complete images consisting of a root file system and kernel. As a first step in a cross-build setup, the framework will attempt to create a cross-compiler toolchain suited for the target platform.