>> P frames are differences from the previous I frame.
>>
>> B frames are differences from the preceding and following frames, whether 
>> they be I or P or B.
>>
>> In other words, you can't play a B frame at all without all the data 
>> between one I frame and the next.
>
> I get that there are full frames and difference frames. I'm just not 
> really understanding why there are two different kinds of difference 
> frames...

I think it's just because P frames are way simpler (both for encoding and 
decoding), so they get their own name.  It's very fixed how the behave (it's 
always the difference from the previous I frame), whereas B frames are much 
more flexible, having inputs from a number of frames ahead and behind (even 
from other B frames, which can make matters very complex).  Some players get 
confused if you use too many B frames, or use them in the "wrong" places, or 
use them at all!

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