Routing define Url pattern and handler information. It is a mechanism to process the incoming url that is more descriptive and give desired response. It is the URL pattern that is mapped together to a handler and routing is responsible for incoming browser request for particular MVC controller. We can also say that routing help you to define a URL structure and map the URL with controller. 

Routing are of two types (after the introduction of ASP.NET MVC5) 

Convention based routing:  We can call MapRoute method and set its unique name, this  url pattern and specify some default values.

Attribute based routing:  In attribute based routing we specify the Route attribute in the action method of the controller.

 There are three segments for routing that are important 

1-ControllerName

2-ActionMethodName

3-Parammeter