[FFmpeg-user] AVFrame structure

[Sense]

In AVFrame structure, there are two fields that I found confusing: key_frame and pict_type. 
For a video frame, what's the difference between them when key_frame=1 and pict_type=I?
My guess is that key_frame=1 indicates that this is an IDR frame(1 for yes, 0 for no), and pict_type=I means this frame is just an I frame(may be or may not be an IDR frame).
Is my guess correct?