ASCII stands for the American Standard Code for Information Interchange, and is pronounced with a hard 'c' sound, as ask-ee. As a standard, ASCII was first adopted in 1963 and quickly became widely used throughout the computer world.

ASCII is a way of defining a set of characters which can be displayed by a computer on a screen, as well as some control characters which have special functions. Basic ASCII uses seven bits to define each letter, meaning it can have up to 128 specific identifiers, two to the seventh power. This size was chosen based on the common basic block of computing, the byte, which consists of eight bits. The eighth bit was often set aside for error-checking functions, leaving seven remaining for a character set.

Thirty-three codes in ASCII are used to represent things other than specific characters. The first 32 (0-31) represent things ranging from a chime sound, to a line feed command, to the start of a header. The final code, 127, represents a backspace. Beyond the first 31 bits are the printable characters. Bits 48-57 represent the numeric digits. Bits 65-90 are the capital letters, while bits 97-122 are the lower-case letters. The rest of the bits are symbols of punctuation, mathematical symbols, and other symbols such as the pipe and tilde.

ASCII began in theory as a simpler character set, using six rather than seven bits. Ultimately it was decided that the addition of lower-case letters, punctuation, and control characters would greatly enhance its usefulness. Not long after its adoption, much discussion arose about possible replacements and adaptations of ASCII to incorporate non-English and even non-Roman characters. As early as 1972 an ISO standard (646) was created in an attempt to allow a greater range of characters. A number of problems existed with ISO-646, however, leaving it by the wayside.

The current leading contender for replacing ASCII is the Unicode character set. This standard allows for essentially unlimited characters to be mapped by using collections of bytes to represent a character, rather than a single byte. The first byte of all Unicode standards remains dedicated to the ASCII character set, however, to preserve backward compatibility.

ASCII is now most often heard in the phrase ASCII art. This describes the use of the basic character set to create visual approximations of images.