What is Poky?

Poky is the Yocto Project reference system and is composed of collection of tools and metadata. Poky is platform-independent and performs cross-compiling, using Bitbake Tool, OpenEmbedded-Core, and a default set of metadata. The main objective of Poky is to provide all the features an embedded developer needs. It is similar to Buildroot which is used to cross compile Kernel or to create RFS for different architectures such as ARM, x86, PowerPC etc. 
Below is the pictorial representation of relationship of Poky, Bitbake, OpenEmbedded-Core

BitBake: 

BitBake was originally a part of the OpenEmbedded project. BitBake is a generic task execution engine that allows shell and Python tasks to be run efficiently and in parallel while working within complex inter-task dependency constraints.

MetaData: 

Metadata is stored in recipe (.bb), configuration (.conf), and class (.bbclass) files and provides BitBake with instructions on what tasks to run and the dependencies between those tasks.

OpenEmbedded-Core: 

Also called as OE-Core,  is a base layer of recipes, classes and associated files that is meant to be common among many different OpenEmbedded-derived systems and forms the basis of the new structure for OpenEmbedded.