Understanding RESTful web services

I've been trying to understand how RESTful web services work and I've come up with a series of doubts and questions for which I have been unable to find an answer. First of all and to make sure I am correctly understanding what this is all about, the following diagram summarizes what a RESTful architecture for a small website could look like:

The first server basically contains a set of services identified by URL's. Those services obtain information from different databases and provide it to someone asking through a REST API (JSON) over http or https. On another server, a web application in an MVC style interacts with those services through their URLS upon user requests.

1. Is my understanding correct?


2. How do web applications such as facebook cope with the delays in getting responses when handling multiple services?


3. How does one make sure that only certain entities are allowed access to services (in this example, how does Server 1 authenticate Server 2)?


4. Is it common to use these kinds of architectures for mobile applications too?


5. When mixing this type of architecture with AJAX, are the AJAX calls done directly to the services or do they go through the main web application controller?