In Lisp, functions are named with
symbols. Smbols are like function names or varible names in other language, althought Lisp allows a wider variety of characters in symbols than most languages allow in their variable names
If a symbol appears in the first position of a list, it's the name of a function to execute. If it appears elsewhere, it's a variable whose value needs to be retrieved.
The way to prevent a symbol (or any Lisp expression) from being evaluated is to quote it by preceding iw with a a single quote(')
Symbols aren't the only things that may follow a ' character; any Lisp expression can be quoted, including lists, numbers, strings, and other kinds of expresssions
In Lisp, strings are
self-evluating, which means that when the string is evaluated, the result is the string itself. So quoting it, while harmless, is redundant. Numbers, characters, and vectors are other types of self-evaluating Lisp expressions
Sexp is an abbreviation for S-expression, which in turn is short for
symbolic expression, which is another name of "Lisp expression"
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