what is the difference between constant class & variable class ?

Let's say you develop a class for your pets.

class Pet {
     var name: String = "Waldo" // just sample initial value
     var type: String = "Hampster" // sample initial value
}

Then you create a variable named myPet . You can also initialize a shelterAnimal.

var myPet = Pet(name: "Gromit ", type: "Dog")  // create an object
var shelterAnimal = Pet(name: "OneEyedPete", "Tabby Cat")  

Because myPet is a variable (var) you can change your pet to another pet, if you wish.

myPet = shelterAnimal  // myPet is a variable, so it can change.

BUT, if you had created myPet as a let....

let myPet = Pet(name: "Gromit", type: "Dog")  // constant
// this next line will fail.
myPet = shelterAnimal  // FAIL. you cannot change the object!

Interestingly, while in the second example myPet is a constant (because of let), you can still change your pet's name. Think of it this way... the body of your pet is the same (it's a constant). But you're able to change your pet's name. Gromit becomes Wendolene.

Paste this into Playgrounds...

class Pet {
    var name: String = "Waldo"
    var type: String = "Hampster"
    // simple class initializer
    init (newName: String, newType: String) {
        name = newName
        type = newType
    }
}

var myPet = Pet(newName: "Gromit", newType: "Dog")
var shelterAnimal = Pet(newName: "One Eyed Pete", newType: "Tabby Cat")
myPet.name  // verify the name

myPet = shelterAnimal // you now have a new pet! (sorry Gromit!)
myPet.name // verify the name.

let yourPet = Pet(newName: "Gromit", newType: "Dog")  // You have a new pet! (you know who...)
yourPet.name = "Wendolene" // and your new pet gets a new name.
// yourPet = shelterAnimal  // Uncomment this line. XCode objects!
yourPet.name // verify the name changed.