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The uninitiated can have trouble figuring out what Yocto actually is. It is an umbrella project/governing group for a set of tools that allow someone to build and use an embedded Linux image.
The developer downloads a version of poky linux which uses bitbake/Open Embedded to download and/or build a toolchain, libraries, a kernel, applications, programs, and images.
From the Yocto web site:
The Yocto Project is an open source collaboration project that provides templates, tools and methods to help you create custom Linux-based systems for embedded products regardless of the hardware architecture.
The main web site is at: http://www.yoctoproject.org/
The Linux Foundation announced the Yocto Project in October, 2010 at Embedded Linux Conference Europe 2010
Version 1.0 was released on April 6, 2011.
Yocto technology derives from Poky Linux, which was a branch of Open Embedded