Amazon AWS and usage model for S3 storage

There is this example on amazon, a high traffic web application. I noticed that they are using S3 as their content delivery method. I was wondering if I need to have a Web Server for the content delivery and a Web App for my application. I don't understand why they have 2 web servers and 2 web app in the diagram.

And what is the best way to set up a website that serves images and static contents through S3 and the rest of the content through the regular storage.

My last question is, can I consider S3 as a main storage, reliable enough that I can only keep my static content there and don't have a normal storage as a backup ?

That is a very general diagram, specific diagrams will vary depending on the specifics of the overall architecture.

Having said that, I believe the Web Server represents something like Apache or Nginx and the App Server represent something like Rails, Rack Server, Unicorn, Gunicorn, Django, Sinatra, Flask, Jetty, Tomcat, etc. In some cases you can merge the Web Server and the App Server together like for example deploying Apache with python mod_wsgi to run your Django app. (So depends on Architecture)

what is the best way to set up a website that serves images and static contents through S3 and the rest of the content through the regular storage.

There's no really best way other than just point your dynamic content to your Databases (SQL and NoSQL) and point your static files to an S3 bucket (images, css, Jquery code, etc) You can also use third party modules depending on your application stack. For example you can accomplish this in Django with the django-storages module. You can find similar modules for other app stacks like Rails.

My last question is, can I consider S3 as a main storage, reliable enough that I can only keep my static content there and don't have a normal storage as a backup ?

S3 is pretty reliable, they provide a 99.999999999% reliability of your data. That goes down if you use their RRS (Reduced Redundancy Storage), but if you want to use it you probably want to back up your data in a non RRS bucket anyways. Anyhow, if it's extremely critical data, you are more than free to backup your data somewhere else just in case.

Notice in the diagram that they also recommend using CloudFront for your static files and this is especially useful if your users will be accessing your application from different geographical areas.

Hope this helps.