Reload only the content area of a website while the sidebars remain static
Yes, it's possible to update the content area without updating the sidebars using Ajax or Ahah. There are multiple modules for implementing ajax functionality in various parts of Drupal:


Views Load More
Ajax pages
Views
jQuery AJAX Load 
etc.


It will depend on the specific requirements of your site and its content as to which method would make the most sense.

Besides Ajax modules, another more advanced option is an idea often referred to as "Headless Drupal" in which you use Drupal as a backend in conjunction with a dynamic front-end framework like Angular or Backbone.

If you're just getting started with Drupal though, I would encourage you to get a bit of experience building simple Drupal sites before getting too focused on Ajax functionality. If you're not already familiar with Javascript, you'll likely end up being somewhat limited in terms of what you can do with the out-of-the-box functionality available from community contributed modules.