For all the methods available in $A, see the JavaScript API at https://<myDomain>.lightning.force.com/auradocs/reference.app, where <myDomain> is the name of your custom Salesforce domain.
A component bundle can contain JavaScript code in a client-side controller, helper, or renderer. Client-side controllers are the most commonly used of these JavaScript resources.
In JavaScript, use string syntax to evaluate an expression. For example, this expression retrieves the label attribute in a component.
var theLabel = cmp.get("v.label");