Box office to collect tickets in Vienna?

I'll go to Vienna next week, and I plan to go and see some concerts there. When I was going to order tickets for the concerts I saw that they are to be picked up at a box office, but I'm not quite sure what that is, and where in Vienna I can find it. Are there lots of box offices, or only one or a few in the city? I'm going to stay at a hotel near the Wiener Prater, does anyone know if there's a box office close to that? I'm grateful for any help. :) Thanks!