ASCII is a character encoding, i.e. a standard set of 128 codes - each equivalent to a 7-bit binary digit - for the letters of a major world language's alphabet, the ten digits 0 to 9, and several punctuation and other symbols.  Variants of this encoding meet the needs of different nations.  The most influential version of these "national ASCIIs" is US-ASCII, which gives codes for most characters needed to write US English.

When used with no other qualification, the term "ASCII" often refers to US-ASCII, which is used widely to encode Internet (web) pages.  However, the place of US-ASCII is gradually being taken over by the 8-bit Unicode character encoding, UTF-8, which includes US-ASCII as the first 128 of its 256 characters.

US-ASCII is also used as the character set of choice for SAMPA, X-SAMPA and CXS.

See also:       The conlang pbwiki article CXS.

On the web:  See the English Wikipedia article "ASCII" at http://en.wikipedia.org/wiki/ASCII.