ASCII is an acronym for American Standard Code for Information Interchange, a widely used standard for encoding text documents on computers. Usually, a file described as "ASCII" does not contain any special embedded control characters; you can view the contents of the file, change it with an editor, or print it with a printer.
In ASCII, every letter, number, and punctuation symbol has a corresponding number, or ASCII code. For example, the character for the number 1 has the code 49, capital letter A has the code 65, and a blank space has the code 32. This encoding system not only lets a computer store a document as a series of numbers, but also lets it share such documents with other computers that use the ASCII system.
Documentation files or program source code files are usually stored as ASCII text. In contrast, binary files, such as executable programs, graphical images, or word processing documents, contain other characters that cannot be normally displayed or printed, and are usually illegible to human beings.
The format of a file, whether ASCII or binary, becomes important when you are transferring files between computers. For example, when using FTP, you can transfer ASCII text files without any special consideration. To exchange binary files, however, you may need to enter the command set binary or otherwise prepare the client to transfer binary files, so that the computer will correctly transmit the special characters in the file.
Note: Most current FTP software will automatically transfer ASCII and binary files correctly.