A terminal symbol is one that can be a part of a string in a formal language. Hwoever, a nonterminal is a “temporary” symbol that is only used to specify the syntax of a language, but it may not appear in an actual string of a formal language. A specific nonterminal, known as the “start symbol”, denotes a string of the formal language.
It is conventional to use Σ or T to represent the set of all the terminals, and N to represent the set of all the nonterminals. For notational convenience, let us use X = Σ ∪ T to represent the set of terminals and nonterminals.
For computer languages, it is a convention to use boldface and teletype font for strings of terminals, such as this. It is conventional to use upper case and italic to represent nonterminals, such as this.
For text-only (without font styles), it is conventional so use quotes to quote strings of terminals, such as "this" or ’this’. Note that punctuations are also quoted.
It is also conventional to use either non-quoted text, such as this, or the use of angle-brackets, such as <this> to represent nonterminals.