What is extension and How to use it?

Ans :
Swift Extension :

Add a new swift file with File > New > File... > iOS > Source > Swift File, but you can call them what you want.
The general naming convention is to call it TypeName+NewFunctionality.swift

Make extension of Double

Double+Conversions.swift

import Swift // or Foundation

extension Double {

    func celToFahren() -> Double {
        return self * 9 / 5 + 32
    }

    func fahrenToCel() -> Double {
        return (self - 32) * 5 / 9
    }
}

How to make extension:

let boilingPointCel = 100.0
let boilingPointFaren = boilingPointCel.celToFahren()
print(boilingPointFaren) // 212.0

Make extension of UIColor

UIColor+CustomColor.swift

import UIKit

extension UIColor {

    class var customGreen: UIColor {
        let darkGreen = 0x008110
        return UIColor.rgb(fromHex: darkGreen)
    }

    class func rgb(fromHex: Int) -> UIColor {

        let red =   CGFloat((fromHex & 0xFF0000) >> 16) / 0xFF
        let green = CGFloat((fromHex & 0x00FF00) >> 8) / 0xFF
        let blue =  CGFloat(fromHex & 0x0000FF) / 0xFF
        let alpha = CGFloat(1.0)

        return UIColor(red: red, green: green, blue: blue, alpha: alpha)
    }

}
See here also.

Using extension :

view.backgroundColor = UIColor.customGreen

Summary : Once you define an extension it can be used anywhere in your app just like the built in class functions. In Objective-C extensions are known as categories.

Objective C Extension : 

In objective c, when you want to make behavior of some property private you use class extension.
-> it comes with .m file only.
-> mainly for properties.
The implementation of the extension must be in the main @implementation block of the file.
Extension can only be added to the classes whose source code is available because compiler compile the source code and extension at same time.