In any computer system but especially in personal computers, a file is an entity of data available to system users (including the system itself and its application programs) that is capable of being manipulated as an entity (for example, moved from one file directory to another). The file must have a unique name within its own directory. Some operating systems and applications describe files with given formats by giving them a particular file name suffix. (The file name suffix is also known as a file name extension.) In general, the suffixes tend to be as descriptive of the formats as they can within the limits of the number of characters allowed for suffixes by the operating system. In DOS 8+3 characters is allowed for a file name.
Almost all information stored in a computer must be in a file. There are many different types of files: data files, text files , program files, directory files, and so on. Different types of files store different types of information.
In a computer, a file system is the way in which files are named and where they are placed logically for storage and retrieval. The DOS, Windows (95-98-Me, NT-2000-XP), OS/2, Macintosh, and UNIX-based operating systems all have file systems in which files are placed somewhere in a hierarchical (tree) structure. A file is placed in a directory (folder in Windows) or subdirectory at the desired place in the tree structure.
File systems specify conventions for naming files. These conventions include the maximum number of characters in a name, which characters can be used, and, in some systems, how long the file name suffix can be. A file system also includes a format for specifying the path to a file through the structure of directories.
File systems limit the size of individual files: the newer the operation system the larger the maximum allowed size.