It's no surprise to most Mac users that you can use arrow keys to navigate through Finder windows. Arrow-key presses work in all three window styles: icon view, list view and column view. What many new Mac users don't realize though is that there are Command versions of these arrow keys.
When viewing folders using icon view or list view, both Command-Up-arrow and Command-Down-arrow play a special role. Command-Up-arrow moves you up to the parent folder of the currently-displayed folder. So if you are in, say, /Users/ericasadun/Pictures/Family, Command-Up-arrow would take you to /Users/ericasadun/Pictures. Press Command-Up-arrow again and you move to /Users/ericasadun. You can check your current folder by command-clicking the icon at the top of the folder window.
Command-Down-arrow only works when the selected item is a folder, and, like Command-Up-arrow, only in icon and list views. Pressing Command-Down-arrow opens and selects the current folder. So it essentially does the opposite of Command-Up-arrow. It moves you further down in the folder hierarchy as opposed to Command-Up-arrow which moves you up the folder tree.
These are useful shortcuts to have at hand when you prefer to use the keyboard to navigate through your files rather than the mouse.