It seems you’ve accidentally installed the GNOME desktop environment. While I personally like it much more than Unity, I can see why you might hate something if it forces its way onto your computer. To get back to Unity (what you had before), you need to switch back to it.
First, however, you need to make sure that
lightdm (the login screen to go along with Unity) is selected as the default to use. To do this, open a terminal (ctrl + alt + T) and run the command,
sudo dpkg-reconfigure lightdm. Put in your password (you won’t see it being typed), and wait for a popup asking you to choose between two or more options. Use your arrow keys to go to
lightdm and hit Enter to select it.
Now that you’re done with that, reboot (a logout may be enough, but I’d rather play it safe). Now you should be at the login screen. Before you login, however, hit the gear icon near the password field (I believe it’s to the right of your username) and choose the
Ubuntu Desktop or
Unity option. I’m not sure which one you’ll see, but it should be one of those.
After you’ve selected Unity/Ubuntu Desktop, login and you should be back to normal.
It looks like you’re running gnome. On the login screen select the DE icon and click Unity, if you don’t get that option, do
sudo apt-get install unity.