Global API provides you global functions to perform common JavaScript tasks such as comparing objects, deep copying, iterating through objects, and converting JSON data etc. All global functions can be accessed by using the angular object. The list of global functions is given below:

angular.lowercase - Converts the specified string to lowercase.
angular.uppercase - Converts the specified string to uppercase.
angular.forEach - Invokes the iterator function once for each item in obj collection, which can be either an object or an array.

angular.isUndefined - Determines if a reference is undefined.
angular.isDefined - Determines if a reference is defined.
angular.isObject - Determines if a reference is an Object.

angular.isString - Determines if a reference is a String.
angular.isNumber - Determines if a reference is a Number.
angular.isDate - Determines if a value is a date.
angular.isArray - Determines if a reference is an Array.
angular.isFunction - Determines if a reference is a Function.
angular.isElement - Determines if a reference is a DOM element (or wrapped jQuery element).

angular.copy - Creates a deep copy of source, which should be an object or an array.
angular.equals - Determines if two objects or two values are equivalent. Supports value types, regular expressions, arrays and objects.

angular.bind - Returns a function which calls function fn bound to self
angular.toJson - Serializes input into a JSON-formatted string. Properties with leading $$ characters will be stripped since angular uses this notation internally.

angular.fromJson - Deserializes a JSON string.
angular.bootstrap - Use this function to manually start up angular application.
angular.reloadWithDebugInfo - Use this function to reload the current application with debug information turned on.

angular.injector - Creates an injector object that can be used for retrieving services as well as for dependency injection

angular.element - Wraps a raw DOM element or HTML string as a jQuery element.
angular.module - Used for creating, registering and retrieving Angular modules.

Models are plain old JavaScript objects that represent data used by your app. Models are also used to represent your app's current state.

A viewmodel is an object that provides specific data and methods to maintain specific views. Basically, it is a $scope object which lives within your AngularJS app's controller. A viewmodel is associated with a HTML element with the ng-model and ng-bind directives.