A-vs-B is an image voting game that involves all eligible images on MCM. You'll be presented with 2 images and by voting for whichever image you prefer - A or B - it is awarded ranking points. The highest ranking images are then displayed around MCM, including on the home page, and below A-vs-B here.
Playing A-vs-B is simple - all you do is click the "Vote A" or "Vote B" button above the image which you prefer. You can also use your keyboards left and right arrow keys to vote. Once voted, another pair will be displayed.