In Sitecore 7 MVC, my understanding is that partial views are represented by using Razor declarations such as `@Html.Sitecore().Placeholder("some_thing")` in a `.cshtml` file. 

In this context, what does the argument `"some_thing"` actually represent - is it something located in the Sitecore instance? If so, how can I locate the thing that `"some_thing"` represents within the Sitecore database?