This article gives an overview of the emergent codecs. People are now utilising the mp4 layer for their file encodings. Read this article to gain this know-how.
.m4a for podcast using the AAC format (may also contain either chapters or images or a mixture)
.m4b for podcasts with chapters and images (as in audiobooks; aka enhanced podcasts)
.m4v for vodcasts using the codec for movie files compatible with video iPods
.mp4 for vodcasts using this general video format
.mov for vodcasts using this traditional video format