In Revit, the Detail Component is an interesting combination of Model and Annotation elements. It is a loadable 2D Family, but unlike a Generic Annotation (which is based on the View scale and will always be the same size on a Sheet), a Detail Component is based on an actual, physical size. But though it is sized according to the model, it is still a detail/ annotation item and so (like Detail Lines) they only exist in the view that they are placed.
Since they are based around actual dimensions, they are endlessly adaptable and can be made highly parametric. Like all other model-based families, they have a Keynote Parameter, which can be used to Tag the element based on a predefined Keynote Database. And since 2014, they have also been schedulable as Detail Items.
Architects tend to use them primarily for Section Details, but there are several other ways that you can use Detail Components in the landscape.