Family

Revit has 3 families:

1. System Family
2. In-Place Family
3. Component Family (Loadable)

System Family

Build with software, cannot be manipulated (create or delete)

Like walls, floors, ceilings, roofs, views, levels ...

The specific element parameters can be manipulated via the creation and application of Type or instance variations
System families are referred as Host elements
A host can receive, support or provide structure for other model elements

In-Place Family

Created directly within the project

Cannot export to other project

You can create in-place versions of many system family categories
Like free-form shape of walls or roofs

You should only consider creating an In-Place Family for elements that are unique to a particular project, and are not reuse for other project

Component Family

Component familes can be host-based or free-standing

Host-based: Need a host, like door, window, lighting ...

Free-standing: No need a host, like furniture