let = Constant and var = variables. Period. As simple as that. Values of constants can not be changed but values of variables can be changed.
Constants and Variables
Constants and variables associate a name with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant can’t be changed once it’s set, whereas a variable can be set to a different value in the future.
Declaring Constants and Variables
Constants and variables must be declared before they’re used. You declare constants with the let keyword and variables with the var keyword. Here’s an example of how constants and variables can be used to track the number of login attempts a user has made:
let maximumNumberOfLoginAttempts = 10 var currentLoginAttempt = 0
In the example above, value of maximumNumberOfLoginAttempts can not be changed but value of currentLoginAttempt can and will be changed as user tries to log in each time.