It looks like you installed another Display Manager somehow (it can happen with a lot of package installations, including other desktop environments). Judging by your background, you installed the default version of Ubuntu. Here's how to get
lightdm (the default login screen/Display Manager) back:
Open a terminal (Ctrl+Alt+T) and run
sudo dpkg-reconfigure lightdm. A window will come up in the terminal. Use your arrow keys to scroll to the
lightdm option and hit Enter. Log out, and you should have the login screen back.
Like David Cole said, the burn-in you see is most likely screenburn, which is hardware related and can't really be controlled by Ubuntu. Follow his link for more info on that. (I'm also pasting it here in case the comment gets removed for some reason: http://lifehacker.com/146469/remove-lcd-image-burn-in)