Final answer:
In JavaScript, 'var' declares a variable with function or global scope, 'let' provides block scope with updating allowed.
Step-by-step explanation:
The difference between let, var, and const in JavaScript pertains to their scope and mutability. The var keyword declares a variable with function scope or globally if declared outside a function, and can be re-declared and updated within its scope.
On the other hand, let provides block scope, which limits the variable's access to the block in which it is defined; it can be updated but not re-declared in the same scope. Finally, const is also block-scoped, but it represents a constant value.
Meaning that once it has been assigned, it cannot be updated or re-declared. Choosing between these keywords depends on the need for mutability and the preferred scope in which the variable should be accessible.