If a disk-based *.avi movie is sent to a video card and from there to a regular TV is that considered streaming or just playback?
Fundamental question :) For me it is a playback.
the difference between playback and streaming is that in streaming you don't have to download the entire file before you can watch/hear it, as in youtube
I have not the same opinion. For me, streaming is when the server feeds its clients (one or more) with a media (video, audio), sent at the good speed for the client to read it. The client just have to decode, with a minimal buffer (needed for network lentency and decoders algorithms) and no needed hard-drive activity. Some video embedded hardware for example doesn't have hard drives and only support incoming streams (like AmiNET or some of them). Often they support MPEG-TS streams provided over UDP at least.
Youtube is a HTTP download of a video file. Like every video file, you can start to read it before the download is ended. You can also have a competitive algorithm which get rid of read data of the file and keep only unread data, you can download slower to minimize your download cache and get closer to streaming. But in this case the client isn't feed with the video, it downloads it.
Link to a mail archive close to "what streaming is":
http://mailman.videolan.org/pipermail/s ... 04254.html
Regards,
Anthony