System.Web.Mvc
The MVC framework is defined through System.Web.Mvc assembly. This is because this is the only assembly which contains classes and interfaces that support the ASP.NET Model View Controller (MVC) framework for creating Web application.
system.web.mvc
Now in vNext (MVC 6), its not dependent on System.Web.Mvc or System.Web anymore . there is Microsoft.AspNet.Mvc assembly which is light for better performance.
System.Web.Mvc assembly