From BlenderWiki
File Browser
The File Browser is used in all the file-related operations. However, it also can be used to browse through the file system without performing any input/output action.
By just using the Editor type select button and selecting File Browser, the actual editor will turn into a File Browser with which you will be able to browse through the file system; however, no input/output action can be performed this way. You can check this by the absence of confirm/cancel buttons on the upper right corner of the editor.
On the other hand, if the File Browser appears as the result of invoking a file action (opening, saving, importing, etc.), it will appear maximized and waiting for an operation to complete before returning to the former screen layout. This time you will be able to see a confirm button (with a text that depends on the operation in course) and a Cancel button.
