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I am following the Agile Web Development with Rails 4 book and I'm a bit confused on a part about rendering. The simple version of the question is... within the application.html.erb file there it says 

render @cart

This is confusing because I thought that there needed to be a controller associated with that view in order to know which partial and @cart variable to use. Is it simply by naming convention that this line looks for a partial like _cart.html.erb? And in that case does it not actually know what @cart is until it renders that partial?

Some clarification would be lovely. Thanks!

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This is a shorthand syntax. From the docs:

Every partial also has a local variable with the same name as the partial (minus the underscore). You can pass an object in to this local variable via the :object option:

<%= render partial: "customer", object: @new_customer %>

Within the customer partial, the customer variable will refer to @new_customer from the parent view.

If you have an instance of a model to render into a partial, you can use a shorthand syntax:

<%= render @customer %>

Assuming that the @customer instance variable contains an instance of the Customer model, this will use _customer.html.erb to render it and will pass the local variable customer into the partial which will refer to the @customer instance variable in the parent view.

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so if there is a :customer in the model, you can user render @customer and it doesn't pass any information and just looks for a partial named _customer?
it looks for the partial named _customer and also passes the instance variable named @customer which is defined from the parent view. In the first example in my answer, they are showing how to pass in a different instance variable, @new_customer, but if you have an instance variable with the same name, you can use the shorthand.
I think application.html.erb is the parent view. With render @cart in the application.html.erb, the variable @cart should change depending on what view is being accessed. So for the index action, @cart would be whatever is defined in the index controller. I'm sort of guessing here, though :)

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