ASCII (American Standard Code for Information Interchange) characters are each represented by one byte according to a standard code. The "low-byte" characters (0 to 127) represent the alphabet and common typewriter symbols, as well as some special screen and printer control codes (such as a page feed). The extended or "high-byte" characters (128 to 255) have several different character mappings, allowing printers to output italics, graphics, international characters, and so on. Thus, only half of the ASCII character set is actually a universal standard. This is referred to as 7-bit ASCII, to distinguish it from the full byte.
ASCII is not designed to represent non-Western alphabets, such as Kanji. There are simply not enough codes to do these symbol sets justice. Various proposals have been put forward to remedy this situation by using two-byte character sets. As of yet, none have been adopted.
The previous discussion is only true for character mode displays. In graphics mode, it is quite possible to display any symbol, though even here a standard would be beneficial.
It should be noted that Microsoft Windows uses the ANSI (American National Standards Institute) character set, which is not fully compatible with ASCII.
See also ASCII files.