As you have probably seen there are a lot more people with button graphics in their clan tags than in previous versions and you maybe thinking why. Before Modern Warfare 3 you had to hack it using a USB Key or similar but now in MW3 you can do it from in the game, which makes it feel less hacky and more acceptable to do. Here’s a video showing how to do all of the different buttons and how to use them to make different phrases. (for Xbox360)
And here is an image of all the buttons and their corresponding codes.