When you are working on a document, it is a good idea to save your work every couple of minutes. In the File menu of the program, you will typically find the options "Save" and "Save As..."
Choosing "Save" simply saves your work by updating the last saved version of the file to match the current version you see on your screen.
Choosing "Save As..." brings up a prompt to save your work as a file with a different name. For example, you might choose to save a document called "Paper - rev. 3" as "Paper - rev. 4". This way, you can save you file at different stages and keep multiple versions on your hard drive.
Choosing "Save" and then "Save As..." is also a good way to make a backup of your file. Just be sure to rename the file something new when you choose "Save As..." or you will overwrite the current saved version, just like choosing the "Save" command would do.