Below are the major components of Angular 2
Modules − This is used to break up the application into logical pieces of code. Each piece of code or module is designed to perform a single task.
Component − This can be used to bring the modules together.
Templates − This is used to define the views of an Angular JS application.
Metadata − This can be used to add more data to an Angular JS class.
Service − This is used to create components which can be shared across the entire application.
</> MV Techbytes 
This blog is very nice.Components are the most basic UI building block of an Angular app. An Angular app contains a tree of Angular components. Angular components are a subset of directives, always associated with a template selenium tutorial for update knowledge on selenium.