(The API reference given here: http://docs.angularjs.org/api is divided into several modules. Each module has it's set of directives, services, filters.
I want to know what do these terms(i.e. directive , service, module etc.) mean and what role will they play in a typical web-app made using angularJS ?
"Service" is explained at https://docs.angularjs.org/guide/services.
Angular services are substitutable objects that are wired together using dependency injection (DI). You can use services to organize and share code across your app.
"Directive" is explained at http://docs.angularjs.org/guide/directive.
Directives are a way to teach HTML new tricks. During DOM compilation directives are matched against the HTML and executed. This allows directives to register behavior, or transform the DOM.
"Module" is explained at http://docs.angularjs.org/guide/module.
Most applications have a main method which instantiates, wires, and bootstraps the application. Angular apps don't have a main method. Instead modules declaratively specify how an application should be bootstrapped.
I'm just including summaries in this answer. You probably will want a lot more detail, in which case follow the links and check out the docs.