Details bridge the gap between the design and building reality, conveying information to the builders and contractors on how a design should be built.
Revit is a building information modeler. You construct a project as a digital representation of the physical objects in the real world. However, not every component needs to be modeled in 3D. Architects and engineers create standard details to represent how materials in the larger project are constructed. Details are vital additions to a project, because they show how materials should interconnect.
There are 2 main types of views you can create for detailing: detail views and drafting views.
When to use details
Foundation detail sketched with Revit detailing tools