Back and Forward
The first two icons are the Back and Forward navigation buttons.

By default, if you are working in a Finder window and click on a folder, Mac OS X will update the navigate to that folder, a new Finder window will not open.  In such a scenario, the Back and Forward buttons will be operational.  The Back button takes you to the previous folder (if any) and the Forward button will only work after you have used the Back button.

View Buttons
The three icons in the middle adjust the views.  There are three ways to view things in Finder, by icon, by list, and by path (also known as column view).

Action Button
The last icon is known as the Action button and is used as a shortcut to invoke actions such as creating a New Folder or to view/change the Finder's Display options.

To use any of the Finder Toolbar cuttons simply click on it (if applicable).  You can even customize the Finder Toolbar to suit your unique computing habits.