An inter frame is a frame in a video compression stream which is expressed as the change from one or more other frames. The "inter" part of the term refers to the use of interframe prediction. In most designs, there are two types of inter frames: P-frames B-frames. The other frame or frames that are used in the prediction of the current frame are referred to as "reference frames". The prediction process ordinarily involves the use of motion compensation. Although the use of the term "frame" is common in informal usage, in many cases (such as in international standards for video coding by MPEG and VCEG) a more general concept is applied by using word "picture" rather than "frame", where a picture can either be a complete frame or a single interlaced field. Video codecs such as MPEG-2, H.264 or Ogg Theora reduce the amount of data in a stream by following key frames with one or more inter frames. These frames can typically be encoded using a lower bit rate than what is needed for key frames because much of the image is ordinarily similar, so only the changing parts need to be coded. 
