The Stationery Pad option has been around for as long as I can remember, in every version of the Mac OS. The Unofficial Apple Weblog does a great job of describing what it does:

If you have any kind of workflow where you need to repeatedly modify some sort of a file template, checking this option in any file's Get Info window will tell its parent application to open a copy of it by default, ensuring that the original file remains unedited.

Surprisingly, very few people know about and use this useful option. To enable it, select a file and open up its Get info window by pressing Command-I. From here you can check the stationery pad option to set that file as a dafault template for its application. Now when you open that application, a copy of the file will load up and you can use it as a template. However, when you save the file it won't save over the Stationery Pad file.

For many applications the icon of the file will change slightly if you set it as a stationery pad. For example, some Microsoft Office files will change to look like a pad.