Folding (also known as reduce or accumulate) is a method of reducing a sequence of terms down to a single term. This is accomplished by providing a fold function with a binary operator, an initial (or identity) value, and a sequence.

There are two kinds of fold: a right one and a left one.

Here's the definition for the fold-right function:

 (define (fold-right f init seq) 
   (if (null? seq) 
       (f (car seq) 
           (fold-right f init (cdr seq))))) 

And here is the definition of fold-left function:

 ;; recursive function (optimized due to proper tail recursion) 
 (define (fold-left f init seq) 
   (if (null? seq) 
       (fold-left f 
                  (f (car seq) init) 
                  (cdr seq)))) 
 (fold-right + 0 '(1 2 3 4)) 
   ; expands to (+ 1 (+ 2 (+ 3 (+ 4 0)))) 
 (fold-left + 0 '(1 2 3 4)) 
   ; expands to (+ 4 (+ 3 (+ 2 (+ 1 0)))) 

A list reversal function can easily be defined using fold-left:

 (define (reverse l) 
   (fold-left (lambda (i j) 
                (cons i j)) 

Further reading: