On Unix systems, and in Unix-affiliated programming languages such as C and Perl, the backslash is used to indicate that the character following it should be treated specially. It is sometimes referred to as the escape character, though this risks confusion with the character generated by the Esc key.
On DOS and Microsoft Windows computer systems, the backslash is used as a delimiter for directory names in file paths. This is in contrast to the use of the slash for this purpose on Unix and other systems. The backslash was chosen for path delimiter because in an early version -- which did not support directories and thus had no need for a path delimiter -- the slash was used to introduce command-line options (which are prefixed by a hyphen ('-') in Unix systems).
In the TeX typesetting system, the backslash begins a markup tag.
In Japanese equivilant to ASCII, the code point that would be used for backslash is instead a yen mark (an upper-case 'Y' with two horizontal lines through it: ¥), while on Korean computer keyboards, the backslash corresponds to the won symbol (an upper-case 'W' with one horizontal line through it: ₩). Nontheless many Japanese environments treat it like a backslash, causing confusion.