Opkg

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.

Build Appliance

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.

Poky

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.