To add to Charles's explanation: typically if you're using Standard English characters and XHTML 1.0 one could suggest placing UTF-8 character encoding within the XML declaration.
ASCII: is a code for representing English characters as numbers, with each letter assigned a number from 0 to 127. For example, the ASCII code for uppercase M is 77. Most computers use ASCII codes to represent text, which makes it possible to transfer data from one computer to another.
Single byte: is usually used in reference to a character set, which supports a maximum of 256 characters. Consisting of 8 bits, one byte (or octet) can support numbers ranging from 0 (zero) to 255, i.e. 256 unique numeric values.
Unicode: is the standard for representing characters as integers. There are seven character encoding schemes in Unicode: UTF-8, UTF-16, UTF-16BE, UTF-16LE, UTF-32, UTF-32BE and UTF-32LE. UTF-16, for example, uses 16 bits per character, which means that it can represent more than 65,000 unique characters. This number has become necessary for some languages, such as Greek, Chinese and Japanese.
Many analysts believe that as the software industry becomes increasingly global, Unicode may eventually replace ASCII (which uses 8 bits for each character) as the standard character coding format.
So you see it's to do with numbers not letters themselves.