What are Files?
All information that your computer handles is stored in files. Files are individual units of data that occupy space on your hard disk. A file can be a document, a picture, a program — anything that is stored and processed by your computer.
When you "open" a file you are telling the computer to go and retrieve and then load into memory that unit of data. When you "save" a file, the computer takes the unit of data in memory and writes it to the hard disk (or a floppy disk of course!).
As far as a computer is concerned, a file is simply an area of hard disk that is referenced by a single name — the name of the file. In order to make this more efficient, the computer divides files into folders (or, more correctly, directories). Within each directory, you can store files. It is possible to store a file called "myfile.txt" in every directory, but a file name must be unique within a directory. You cannot have two "myfile.txt" files in the same directory as the computer would not know which one you were referring to.
All files are stored in binary — that is, 1's and 0's. The program you are using will determine how those 1's and 0's are treated. A paint package will assume that the file is a picture, while a word processor will think it is a document. Some files are not meant to be "displayed'" and if you try to do this, you will "see garbage." This is because the file contains 1's and 0's that could well be computer instructions. This is a program (like a word processor, paint package).
So how does the computer "know" what is and is not a valid file to "open"?
File Types
This is handled in two ways. First, there are "File Types." A file type is an (up to), 3-character extension that appears after the dot of a file name. In the example above — myfile.txt, the name of the file is "myfile" and the extension is "txt." So that the computer knows where the name ends and the extension begins, they are separated by a dot — hence myfile.txt. Different programs will use different extensions in order to determine the type of file. This, on its own, is not good enough as there is nothing to stop you naming, say, a text file with a picture file extension. Like renaming myfile.txt to myfile.bmp. If your paint program tried to open myfile.bmp the text simply isn't a picture and rather than attempting to open the file as a picture, you will get an error message. How does the computer know this? This is achieved by using header records.
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