These are the individual projects that make up the Yocto Project. Each of these holds a key role in developing embedded Linux, and the Yocto Project works to make all of them interoperable and useful.
Opkg is a lightweight package management system. It is written in C and resembles apt/dpkg in operation. It is intended for use on embedded Linux devices and is used in this capacity in the OpenEmbedded and OpenWrt projects, as well as the Yocto Project.
As best it can, opkg maintains backwards compatibility with ipkg and conforms to a subset of debian's policy manual regarding control files.
The Build Appliance is a virtual machine image which enables you to build and boot a custom embedded Linux image with the Yocto Project using a non-Linux development system. We don't recommend that you use the Build Appliance as a day-to-day production development environment. You should limit the Build Appliance to testing out and experiencing the Yocto Project. The Build Appliance leverages other projects in the Yocto Project, particularly Hob, which boots inside the virtual machine.
AutoBuilder is a project to automate build tests and QA.
OpenEmbedded Core contains base layer of recipes, classes and associated files that is meant to be common among many different OpenEmbedded-derived systems, including the Yocto Project. This set of metadata is co-maintained by the Yocto Project and the OpenEmbedded Project.
The Eclipse IDE Plug-in integrates the functionality of the Yocto Project ADT and toolchain into the Eclipse IDE.
Poky is a reference system of the Yocto Project. It contains the OpenEmbedded Build System (BitBake and OpenEmbedded Core) as well as a set of metadata to get you started building your own distro. To use the Yocto Project tools, you can download Poky and use it to bootstrap your own distribution.