In math: A string is a finite sequence of symbols that are chosen from a set called an alphabet.
In CS: A string is a sequence of characters.
Strings in the way computers represent textual information.
Most problems with strings have to do with search a string or a list of strings/patterns in another string.
In computer memory, strings are nothing more that binary data.
Data in computer memory
In order to interprect what the value 0x48
(first 8 bits) means in terms of characters, we need an encoding.
The problem of encoding predates computers. For example, Morse code is an encoding between short and long sounds to Latin script.
ASCII was originally a 7-bit encoding.