I have loved Pentatonix ever since they competed on (and won!) The Sing-Off a few years ago, but I hadn't heard anything about them since. Then this video came out and I remembered how much I loved them! Their Christmas album actually came out last year (as did this song), but it's becoming very popular again as a result of this song.
I hope you love it as much as I do! I bought the whole album (it's only $8 on Amazon/iTunes) and have been enjoying it since Wednesday. :)