How do I get the original finder view back? For some strange reason, Finder has changed to a different layout and I can't figure out how to revert it to the standard one.

This is what it looks like now:



How do I revert it to the original, default layout, that has folders, drives and devices in the left-most column and all the other files and folders in the middle panes? 

I see the original layout when I go to upload a file here, so you can see the screenshot here:



How do I get back that original layout and prevent it from changing again? This is the second or third time it has changed.
Show the Toolbar with ⌥⌘T or View → Show Toolbar
Show the Sidebar with ⌥⌘S or View → Show Sidebar