What is the $A object in lightning?
Yes, $A is how you gain access to the underlying Aura framework, such as enqueuing actions, getting application events, managing the Aura rendering life cycle, and various utility methods. You can view the available methods using AuraDocs, located on your server at /auradocs/reference.app. You can access it using the production or sandbox links I've provided here. You'll find the full list in the JavaScript API section.
The Aura object is the top-level object in the JavaScript framework code. $A is shorthand for Aura.