Poky is the reference distribution of the Yocto Project, used as something which can be tested and put through QA and that has a regular well established six month release cycle. One of the cornerstones of the Yocto Project is that there should be something tangible which people can look at that has a known quality and start from. Poky provides this.
At the technical level it is a combined repository of the components BitBake, OpenEmbedded-Core, meta-yocto and documentation all together in one place provided, known to work well together. Meta-yocto is intentionally small having some hardware reference BSPs (so tests can be made on real hardware) and a small amount of "distrubtion policy".
More information can be found at http://www.yoctoproject.org/.
Historically, Poky was a form of parallel evolution of the main OpenEmbedded tree, being smaller/cut down and more focused on specific targets. It was sponsored heavily by OpenedHand with Richard Purdie as its architect and lead developer and has been referrred to as a build system in its own right. To be clear it always used BitBake and the OpenEmbedded architecture so the build system/approach was always OpenEmbedded. With the creation of the Yocto Project and the refactoring of OpenEmbedded into several components such as OpenEmbedded-Core and Meta-OpenEmbedded, Poky became the reference distribution as many of the reasons for the original creation of Poky were addressed by those changes.