How to use unity scripting api?

How we use unity scripting API when programing?is API like standard library ? How API work? example how to use it?

The API contains all the methods to achieve anything through code in Unity. It essentially is Unity - certainly from a programmer’s perspective.
https://unity3d.com/learn/tutorials

Application program interface (API) is a set of routines, protocols, and tools for building software applications. An API specifies how software components should interact. Additionally, APIs are used when programming graphical user interface (GUI) components. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

Best explanation of API https://en.wikipedia.org/wiki/Application_programming_interface

MonoBehaviour is the API, MonoBehaviour is the base class from which every Unity script derives.

Edited: (thanks to @Hellium)

When you use C# “as unity script to interact with unity gameobjects”,
then you must explicitly derive from MonoBehaviour.

Yes you do not need all of c# classes to be drived from MonoBehaviour (When you do not need to interact with GameObjects) When you use UnityScript (a type of JavaScript), you do not have to explicitly derive from MonoBehaviour.

Note: There is a checkbox for disabling MonoBehaviour on the Unity Editor. It disables functions when unticked. If none of these functions are present in the script, the Editor does not display the checkbox. The functions are:

  • Awake()
  • Start()
  • Update()
  • FixedUpdate()
  • LateUpdate()
  • OnGUI()
  • OnDisable()
  • OnEnabled()

Basically we designed API in the way that a programmer use its public methods in his script and all private members are hidden from the user, we call only API and get our desired result without doing anything.

Lets take an example we want to use String API Represents text as a series of Unicode characters.

Unity uses the .Net System.String class for strings.

Note: In c# string is an alias for System.String. This means that you can use either string or String in your code (if you have added using System to the top of your script.)
Every component has its own API in the unity engine.

thankyou. really detail, helpful.