ASCII
A standard for encoding English upper and lower case letters, numbers, and special characters. (ASCII = American Standard Code for Information Interchange).
ASCII uses only seven bits (seven characters per byte or word) and 128 characters. The most common standard for simple text files (such as .txt and .csv).
To find the ASCII equivalent of any letter (either decimal or hexadecimal), you can look it up in the ASCII Table (for example, the letter A equals ASCII decimal code 65 and the number 7 equals decimal code 55).
To provide more characters, such as accented characters and mathematical symbols, an 8th bit is often added for a total of 256 characters (2 to the 8th power).
There are different standard 256 character sets, but the most common is ISO Latin 1 (Western European) which is the default character set on most browsers and keyboards. Other sets include: Greek, Arabic, Cyrillic, and Hebrew.
See Unicode