I don't know a huge amount about video formats, but I do know there are codecs that do the actual encoding and containers, which specifies and contains both the audio and video file format(codecs) and contain extra information, such as subtitles (if supported). h.264 is a codec, avi, mp4, wmv and flv are containers.
The Roku can play h.264 encoded files, at least if encoded to specific specs (I don't have details on that, but check this forum), but it can only stream from certain containers. The definitive list is in the SDK docs.
What I understand about flash is that it can play whatever container/codec combinations are supported by the flash binary. Adobe would have the definitive list. Wikipedia does have a nice table though:
http://en.wikipedia.org/wiki/Comparison ... er_formats
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