The profile determines what the codec is allowed in terms of color space, data rates etc.
They basically are different encoding levels setup to meet a certain set of specifications of intended applications or playback devices. So a lot of the features or capabilities of H.264 are not possible in the majority of codecs used for the web.
For the iPhone and mobile apps, you have to use the lower level profiles since they have tiny and slow processors and the data rate capabilites of celluar data is slow.
There's a more accurate explanation and description of the many profiles here:
http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC#Profiles
Chris Blair
Magnetic Image, Inc.
Evansville, IN
http://www.videomi.com
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