Chrome Extension as GUI for JSON API

Chrome Extension as GUI for JSON API

I'm working on kind of huge project using PHP which is a CRM and Smart
Ticketing and Processing System for travel agencies and I'm planning to
make it only available on chrome.
And I'm new to Chrome Extensions/Apps, But when i'm reading about creating
extensions i surprised that i can create any html content with any js
codes included.
My questions are:
Can I include my application GUI with pure HTML && CSS && JavaScript in
some extension and make it send/receive requests to my PHP-API Which
already exist in my code since it's fully ajax project?
And if this is able to be done what do i have to create Google-Chrome-App
or Google-Chrome-Extension And what is the difference between them and
where to start reading about required solutions.
If this can be done it will be perfect since the GUI elements (HTML, JS,
CSS, Images, etc..) will be loaded from local store machine of each
user/employee's machine and the only transfered data will be the JSON
requests.