ASCII is a character set that was created in the 1960s in the US. ASCII is an acronym for the American Standard Code for Information Interchange. It was based on telegraph code and each character is represented by 7 bits. Using 7 bits means that ASCII can represent 128 characters (lowercase, uppercase, numbers, a range of symbols, and control characters). The order of characters in the ASCII character set follows a logical order: e.g. A (65 in denary) is followed by B (66 in denary) so it it possible to deduce the binary representation of characters if you have some reference.
Limitations of ASCII
With only using 7 bits, 128 characters can only cover characters within the English language. As more countries began to use computers and interest grew in being able to use them to communicate there became a need to represent a larger number of characters. ASCII Extended was created to make use of 8 bits. This allowed 256 characters to be used and included a range of additional Latin characters including those with accents.