ArcGIS REST API vs JavaScript API

[RJ]

Am new to ArcGIS and still learning.   My question is what is the difference between using REST API vs JavaScript API?

Given an ArcGIS Server and availability of these API's and say for example I need to provide a way for the user in my web application to draw a polygon on the map, how do I do that using REST API?. 

Based on what I have read so far, JavaScript API provides a draw toolbar that can be used along with the related JS API however unclear on how REST API can be used.

[RichardWatson]

The JavaScript API builds on, i.e. utilizes, the REST API.  In fact, all ESRI clients provide language/environment specific bindings (e.g. SilverLight, Flex, WPF, JavaScript, iOS, Andriod, etc.) which use the REST API.

If you are developing in JavaScript then you should strongly consider using the JavaScript API otherwise you have a lot of plumbing to do.

Why would you not use one of the ESRI APIs?

- You are running in an environment for which no API is available, e.g. an IBM mainframe.
- The licensing terms and conditions of using the APIs are, for whatever reason, unacceptable to you.

[RJ]

Thank You Richard.  Makes sense.  In our case using JS is a viable option that we can definitely consider.