Quality/Keyframes . In order to compress a movie for Web distribution, codecs remove redundant data from a movie with one or both of two methods:

• INTRA-FRAME : Often called Spatial Compression , this method removes redundancies within a single frame of video. Each frame of video is still discrete and can be accessed as a unique point in time. M-JPEG is a prime example of an Intra-Frame Codec.
• INTER-FRAME : Often called Temporal Compression , this methods removes redundancies across a group of pictures. MPEG, Sorenson and RealSystem are Inter-Frame Codecs.

INTRA-FRAME or SPATIAL COMPRESSION: The contents of an individual frame are analyzed for redundancy and then compressed. Areas showing flat color or with no motion can benefit from compression within the space of a single frame. When selecting a codec that uses Spatial Compression, you are often given a choice of the degree of compression. It is usually expressed as a " quality slider ."

INTER-FRAME OR TEMPORAL COMPRESSION: The contents of a group of pictures or scenes within a movie are analyzed for redundancy between adjoining frames across entire sections of the movie...across time.

A " keyframe " is used in Temporal Compression in order to instruct the codec to stop looking for redundancies across frames and to refresh itself with all new data from the movie.

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