Key Schedules and Building Component Schedules

In Revit there are two distinct schedule types, “Building Components” and “Key” schedules.

Building Component Schedules
These are the default schedule types, each object instance will appear in the schedule by default.
Key Schedules
Best used to setup a number of standard “styles” for an object type.  Each row of data in the Key Schedule will create a new style to apply to an object.  Once applied, the style will pre-fill all fields as needed.  This setup is ideal where there are numerous standard types, and you don’t need every instance of the object to appear in a schedule.  I have included a short video below on this process:
Video Example
A few items to keep in mind:
1.  Key Schedules cannot utilize Shared Parameters for a field; “Project Parameters” are only available in this schedule type.
2.  To combine “Keys”, “Project Parameters” and “Shared Parameters” in a schedule, use a “Building Components” schedule type.  I have included a short video below on this process:
Video Example
I hope this information is helpful to better explain the two schedule types.
Source: revitclinic.typepad.com