What is the difference between convenience init vs init in swift, explicit examples better

Question

I am having troubles to understand the difference between both, or the purpose of the convenience init.

Answer 1

All answers are sounds good but, lets understand it with an simple example

class X{                                     
   var temp1
   init(a: Int){
        self.temp1 = a
       }

Now, we know a class can inherit another class, So

class Z: X{                                     
   var temp2
   init(a: Int, b: Int){
        self.temp2 = b
        super.init(a: a)
       }

Now, in this case while creating instance for class Z, you will have to provide both values 'a' and 'b'.

let z = Z(a: 1, b: 2)

But, what if you only want to pass b's value and want rest to take default value for others, then in that case you need to initialise other values a default value. But wait how?, for that U need to set it well before in the class only.

//This is inside the class Z, so consider it inside class Z's declaration
convenience init(b: Int){
    self.init(a: 0, b: b)
}
convenience init(){
    self.init(a: 0, b: 0)
}

And now, you can create class Z's instances with providing some, all or none values for the variables.

let z1 = Z(b: 2)
let z2 = Z()