gstreamer/docs/design/part-caps.txt

Caps
----

Caps are lighweight refcounted objects describing media types.
They are composed of an array of GstStructures.

Caps are exposed on GstPadTemplates to describe all possible types a
given pad can handle. They are also stored in the registry along with
a description of the element.

Caps are exposed on the element pads using the _get_caps() pad function.
This function describes the possible types that the pad can handle or
produce (see part-pads.txt and part-negotiation.txt).

Various methods exist to work with the media types such as subtracting
or intersecting.