If you're running from a live CD/DVD/USB and have not yet installed Ubuntu, and you clicked Log Off, that's not part of the installation. It's not asking you what username and password you want to use in the system you're installing. It's asking for the username and password for the live environment.

As AliNa says, the username is ubuntu and the password is blank (assuming you have not changed it in the live CD/DVD/USB since you booted from it, which you probably have not).

If for some reason you want to shut down on the login screen (for example, if that username and password doesn't work), holding down the power button is not the best way to do it. A very clean way to restart is to press Ctrl+Alt+F1 followed by Ctrl+Alt+Delete.

But you should be able to log back in to the live CD/DVD/USB system with the username ubuntu and the blank password.

If you're running from a live CD/DVD/USB and you want to install Ubuntu, and the installation is not started, you probably selected Try Ubuntu without installing rather than Install Ubuntu. If that's the case, then when you're ready to install, you can click the Install Ubuntu icon on the Unity launcher, which is the bar of icons on the left side of the desktop.

Unable to shut down laptop after Logging Out of Ubuntu 12.04
I've recently watched a BBC documentary about the history of music and recall an episode where they talk about music that has two distinct tempos (and possibly time signatures) at the same time for different sets of instruments.

Still, I can't remember how this is called and who are the famous composers that composed that way (Stravinsky?).
