Here is a list of all files with brief descriptions:

src
libtheora-1.2.0
include
theora
codec.h	The shared libtheoradec and libtheoraenc C API
theora.h	The libtheora pre-1.0 legacy C API
theoradec.h	The libtheoradec C decoding API
theoraenc.h	The libtheoraenc C encoding API