ASCII is the American Standard Code for Information Interchange.
ASCII codes are the 8 bit patterns of 1's and 0's (binary numbers) that represent text in computers and other devices that use text.
For instance the letters 'A' and 'a' is represented by:-
A = Decimal 65, hex:0101, Octal 0x41
a = Decimal 97, hex:0140, Octal 0x61
With numbers :-
0 = Decimal 48, hex:0060, Octal 0x30
9 = Decimal 57, hex:0071, Octal 0x39
And this means that if you subtract the ASCII character 0 (decimal 48) from the ASCII character 9 (decimal 57) you get the number 9. This is how keyboard keystrokes which are captured as text (ASCII) are converted into numbers in the computer when you need to do sums (e.g. in a spreadsheet).
There are other encodings that may also be used such as Extended Binary Coded Decimal Interchange Code (EBCDIC), but ASCII is the most common.
As a point of interest, with Roman character sets (which have about 127 characters) it is possible to encode most of the characters needed using just 8 bits (there is therefore room for 255 characters). However as use of computers spread and it became necessary to provide for different scripts such as kanji, which have many more characters to encode (the Yiti Zidian dictionary published in 2004 contains 100,000 or more individual characters) the character set used in computers and by programming languages had to be extended (by grouping bit patterns) to cope. The result is that you can type using kanji utilising a computer with a Chinese/Japanese keyboard. (Although how anyone can learn/remember 100,000 different characters is beyond my comprehension!).
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ASCII code is a special code used to represent the entered data to the computer. This means when I press letter "A" on my keyboard, the keyboard takes the ASCII code and goes to the processor . After that the processor gets the ASCII code of the entered letter it understands that the entered letter was "A". In simple world we can say it is the way that letters appear for the processor.
ASCII reserves the first 32 codes (numbers 0-31 decimal) for control characters: codes originally intended not to represent printable information, but rather to control devices (such as printers) that make use of ASCII, or to provide meta-information about data streams such as those stored on magnetic tape. For example, character 10 represents the "line feed" function (which causes a printer to advance its paper), and character 8 represents "backspace". RFC 2822 refers to control characters that do not include carriage return, line feed or white space as non-whitespace control characters.[34] Except for the control characters that prescribe elementary line-oriented formatting, ASCII does not define any mechanism for describing the structure or appearance of text within a document. Other schemes, such as markup languages, address page and document layout and formatting.
The original ASCII standard used only short descriptive phrases for each control character. The ambiguity this caused was sometimes intentional (where a character would be used slightly differently on a terminal link than on a data stream) and sometimes accidental (such as what "delete" means).
so all compuers that use ascii can transmitt charictors to each other and understand what they mean