BDAV refers to either the transport stream format used for all Blu-ray content, or the plain format intended for home video with no interactivity.
The BDAV Container format is based on the standard MPEG-2 TS (transport stream) Both Blu-ray and HD DVD use transport streams, compared to DVD's program streams, to store video, audio, and other streams. This allows multiple video programs to be stored in the same file so they can be played back simultaneously, giving a Picture In Picture effect.
The BDAV disc format is the consumer oriented alternative to the BDMV discs made by professional Authoring houses for movie releases. Although early Blu-ray players were released with Firmware allowing playback of non-AACS encrypted content on BDMV discs, current Blu-ray specifications will result in that feature being removed, and BDAV discs being the only unencrypted Blu-ray format supported on players.
BDAV was designed in part to provide compatibility between AVCHD digital camcorders and Blu-ray players. It's comparable to the DVD formats used for standalone DVD recorders, DVD+VR and DVD-VR because it's designed primarily for authoring simple Video/Audio content with no menus.