A file in a computer system is a stream of bits stored as a single unit, typically in a file system on disk or magnetic tape.
While a file is usually presented as a single stream, it most often is stored as multiple fragments of data at different places on a disk (or even multiple disks). One of the services operating systems usually perform for applications is that organization of files in a file system.
Files are created by software and usually conform to a particular file format. They are almost always assigned file names by the file system on which they are stored, so that they can be refered to at a later time.
An important subclass of file is the text file. A text file is a sequence of characterss often organized into lines separated by line breaks. The term "binary file" usually refers to any file other than a text file.
A "special file" is a file system object which is accessed as though it was a file, but the sequence of bits is supplied or consumed by another process (or by the operating system itself) such as a device driver or network interface. Indeed, the philosophy that "everything is a file" is one of the best known desing decision in Unix and Unix-like operating systems (such as Linux).
Files are often organized hierchically by the operating system, placing them in folders or a directories.