poky

What is Poky?
  Poky: Poky is a "Reference distribution".

What is Reference distribution?
  Reference distribution: A `Distro` that provides a bitbake to compile All of your kernel,bootloader.and minimal packages for
  rootfs.

Examples: poky , yogurt, baryon,luv,luv-netboot,micro, oe-measured,yogurt-wayland e.g.,

Bydefault yocto provides poky Distro.and this will have different versions. 
Follow link for yocto versions "https://wiki.yoctoproject.org/wiki/Releases"

you can create your own distro wirh reference to poky.
examples: poky-atmel, poky-lsb, poky-tiny,poky-sota,poky-edison e.g.,

yogurt is also a Distro like poky
examples; yogurt-tiny, yogurt-vendor, yogurt-wayland. some people will take yougurt distro and create their own distros

so some people will use poky distro and some people will use togurt ...etc.,

After this distro you will create your own meta-layer for your board like phyboard-wega, wand board, beaglebone, Raspberry-pi,nitrogen boards, etc.,

open-embedded core(meta-oe) is a main layer which gives more packages for your rootfs.
examples: meta-network, meta-python, meta-ruby etc.,

                                        YOCTO
                           _______________|__________________
                           |              |                  |
                       Distro           Layers              Recipes
                       poky         eg: meta-phytec       e.g: linux_version.bb
                       yogurt           meta-acer               gcc_version.bb
                       X11              meta-altera             uboot_version.bb
                       wayland          meta-Qt5                fackroot_version.bb
                       
Note: follow link for all yocto DISTRO'S , LAYERS, RECIPES.