This is a far more advanced variation of save that can be also accessed from keyboard by pressing Alt-F -> "A" keys. You have probably been using it to save additional copies of your document – saving the file under a different name or in a different directory. Please, note the "quick access" options on the right – there is a link to several directories: the desktop, your recent documents, the "My documents" folder and "My Network Places".
Also note the easy ways to navigate through your folders. Creating a new folder is also possible right from this window.
Of course you can also select one of your existing files and save under its name, overwriting the old content, but there is more. The saving formats are very important and may deserve a separate article.