How to declare a string type ?

Question

I am unable to understand the difference between a constant data type and a string data type.

Answer 1

A constant is not a type of data but it declares that a certain variable's value can't be changed after its first set.
To declare a constant use the "let" keyword:

let PI: Double = 3.14
let MY_NAME: String = "Gleb Golov"

See that I annotate different constants with different types.
Here are few very variables (their value can be changed, assigned to another value):

var year: Int = 2016
var actualPresidentName: String = "Donald Trump"

// Jump 4 years!
year += 4
// Now it's 2021
// Time to change presidency!
actualPresidentName = "Mickey Mouse"

Answer 2

Mickey Mouse nice one , thanx Gleb