What is ' (apostrophe) in Lisp / Scheme?

Question

I am on day 1 hour 1 of teaching myself Scheme. Needless to say, I don\'t understand anything. So I\'m reading The Little Schemer and using this thing:

Answer 1

You need to understand the basic evaluation rules of Scheme.

First:

(atom? 'turkey)

The list is a function application, so atom? gets evaluated to a function. 'turkey is a short hand notation for (quote turkey). Evaluating (quote turkey) gives the symbol turkey.

So next the function is applied to the symbol turkey and a return value is computed.

Second

(atom? turkey)

Again we have a function application and atom? gets evaluated to a function. This time turkey is a variable. Evaluating turkey gives the value that is bound to it - what ever it is.

So then the function is applied to the value of the variable turkey.

Summary

turkey is a variable, which gets evaluated to its value. 'turkey is (quote turkey), which gets evaluated to the symbol turkey.

Scheme reuses s-expressions and builds its programs out of s-expressions. This leads to the problem that sometime turkey should be a variable and sometimes it should be the symbol. This is slightly confusing for the beginner. After some time you'll see the power behind it.