Scheme: When to use let, let*, and letrec?<>
This question already has an answer here:
Your best bet is to read the official R5RS descriptions of let, let*, and letrec.
In short, however:
(let ((x 2)) (let ((x 3) (y x)) y) => 2 (let ((x 2)) (let* ((x 3) (y x)) y) => 3
So the difference between let and let* is let will evaluate all the bindings with respect to the level above (so it doesn't matter what order they're listed in) while let* does it sequentially. (let* ((x a) (b y))) is equivalent to (let ((x a)) (let ((b y))).
Letrec, on the other hand, allows you to bind recursive values. So you might write a recursive function that you only want to be within the function scope and bind it to a name using letrec.