A filter is, essentially, a program that operates on a stream of characters. The source and destination of the character stream can be files, another program, or almost any character device. The transformation applied by the filter to the character stream can range from an operation as simple as character substitution to one as elaborate as generating splines from sets of coordinates.
The standard MS-DOS package includes three simple filters: SORT, which alphabetically sorts text on a line-by-line basis; FIND, which searches a text stream to match a specified string; and MORE, which displays text one screenful at a time.