What is the difference between a regular method and an extension method in Kotlin?

A regular method is declared inside a class and operates on the data of that class.

An extension method allows you to "add" a method to an existing class, including third-party libraries, without modifying its source code. It is declared outside the class and has a special syntax: the name of the extended type, a dot, and the method name.

class MyClass {
    fun regularMethod() {
        // Operates on MyClass data
    }
}

fun MyClass.extensionMethod() {
    // Has access to this - the instance of MyClass
}