Let: Calculates the expression assigned to it and sets the expression result to the variable.

Set: Assigns the value(or expression which is after the equal sign) as it is without compute.

Example:

Set vVar=2+3; //vVar has the expression 2+3 as value we have to compute it using                                                  $(vVar) which returns 5

Let vVar=2+3;//vVar has the computed value 5