It's where ATRL users (normally those that have Countdowns in 'Your Top Ten', and among many others) come together and reveal their Top Songs/Singles/Albums/Artists/etc of the Year and make Year-End Lists/Countdowns.
Most people, (including me) make Year-End lists from Last.fm stats while some others do things such as lists they made themselves of their favorites. If you wanna do something like your own Year-End Top 100 Songs of 2013 then if you have a Last.fm use it (scrobbles from Last 12 Months) and do your Most Played Songs.
Also as a tip for those making lists, I know I use multiple things for music (iTunes/Spotify/etc) so some songs can have multiple scrobbles under different names ("#Beautiful", "#Beautiful - feat. Miguel", "Beautiful (feat. Miguel)", etc) so be sure to press CTRL+F for each song and type it's name and add the plays from all names.
I can tell it's going to be hard for me to find time for BO2013 this year but I'll get something out there. Maybe a Top 100 with not-as-long write-ups.
:
..but with the way the server is going no thank you