A filename is a text string that identifies a file. Every file stored on a computer's hard disk has a filename that helps identify the file within a given folder. Therefore, each file within a specific folder must have a different filename, while files in different folders can have the same name.
Filenames may contain letters, numbers, and other characters. Depending on the operating system, certain characters cannot be used since they conflict with operators or other syntax used by the operating system. Different operating systems also have different limits for the number of characters a filename can have. While older operating systems limited filenames to only 8 or 16 characters, newer OS's allow filenames to be as long as 256 characters. Of course, for most practical purposes, 16 characters is usually enough.
Filenames also usually include a file extension, which identifies the type of file. The file extension is also called the "filename suffix" since it is appended to the filename, following a dot or period. For example, a Microsoft Word document may be named "document1.doc." While technically the filename in the preceding example is "document1" and "doc" is the extension, it is also acceptable to refer to "document1.doc" as the filename. In some cases, the filename may even refer to the file's directory location, i.e. ("\My Documents\School Papers\document1.doc").
You can name a file by clicking on the file's icon or filename, waiting for a second, then clicking on the filename again. As long as the file is not locked, the filename will become highlighted, and you can type a new name for the file. You can also name a file the first time you save it from a program or by selecting "Save As..." from the program's File menu.