MVC 6 View Components vs. Partial Views: What is the difference? When should each be used?

According to this link- https://docs.asp.net/en/latest/mvc/views/view-components

New to ASP.NET MVC 6, view components (VCs) are similar to partial views, but they are much more powerful. VCs include the same separation-of-concerns and testability benefits found between a controller and view. You can think of a VC as a mini-controller—it’s responsible for rendering a chunk rather than a whole response.

So it just a enhancement of partial view and another difference is when you use partial view you still have dependency on controller while in View Component you don't need a controller. So there is a separation of concern.

There is a detail post for ASP.NET View Components. http://www.tugberkugurlu.com/archive/exciting-things-about-asp-net-vnext-series-mvc-view-components

An example where you might want to use a ViewComponent as opposed to a PartialView:
You need to write a bunch of business logic where for example you might need to contact a 3rd party web service and get the data and do something with it and then display this information.

For the above scenario, sure you could write C# code in the partial view itself, but its ugly and also you want the code to be testable. So this is where a view component can be useful, i.e you can write all your business logic within a view component and return a view (this is of type ViewViewComponentResult).

View components are NOT the same as child actions.

ViewComponents are also used when you need a partial view, that requires a model to be called in _Layout. To avoid writing C# code to create the model in the Layout, it's best to use a ViewComponent that can make use of the Services configured for the application, same as controllers, through dependency injection.