I am always amazed how only four guys with their mouths and no instruments at all can do such wonderful songs. They certainly know how to sing. So if you have some 50 minutes available, here is it. It starts with a German song, but they also translated some of their original songs to English (which is difficult I guess, the lyrics often are very funny and have a twist to them), as well as singing English covers. The medley at the end is a retrospect of the famous songs of the year 2013, they do that every year.
With their own songs, there is usually much more choreographed dancing and show as well.
The small guy with the glasses is the one that writes all the songs, and he is a genius. At German concerts, there is a part where the audience shouts some random words and he makes an improvised song out of them right there on stage.
And, just because I can, here is the John Barrowman connection: He did a concert in the same venue back in 2002, long before he became famous with Doctor Who. The first part can be watched here, the other links are at the side: