Here is a list of all documented files with brief descriptions:

vpx
vp8.h	Provides controls common to both the VP8 encoder and decoder
vp8cx.h	Provides definitions for using VP8 or VP9 encoder algorithm within the vpx Codec Interface
vp8dx.h	Provides definitions for using VP8 or VP9 within the vpx Decoder interface
vpx_codec.h	Describes the codec algorithm interface to applications
vpx_decoder.h	Describes the decoder algorithm interface to applications
vpx_encoder.h	Describes the encoder algorithm interface to applications
vpx_ext_ratectrl.h	Defines structs and callbacks needed for external rate control
vpx_frame_buffer.h	Describes the decoder external frame buffer interface
vpx_image.h	Describes the vpx image descriptor and associated operations
vpx_tpl.h	Describes the TPL stats descriptor and associated operations