A key frame in animation and filmmaking is a drawing that defines the starting and ending points of any smooth transition. The drawings are called "frames" because their position in time is measured in frames on a strip of film. A sequence of keyframes defines which movement the viewer will see, whereas the position of the keyframes on the film, video or animation defines the timing of the movement. Because only two or three keyframes over the span of a second do not create the illusion of movement, the remaining frames are filled with inbetweens.
Traditional animation 
In the workflow of traditional hand-drawn animation, the senior or key artist would draw the keyframes, then, after testing and approval of the rough animation, give the scene to his assistant. The assistant does the clean-up and the necessary 'inbetweens', or, in really big studios, only some breakdown which define the movement in more detail, then give the scene to his assistant, the 'inbetweener' who does the remainder.
Animation by means of computer graphics 
Use of key frames 
In computer animation the workflow is basically the same. The animator creates the important frames of a sequence, then the software fills in the gap. For example, in Adobe Systems Flash, the animator can specify, in keyframes, the starting and ending position of an object, such as a circle. Flash smoothly translates the object from the starting point to the ending point. Similar to hand-drawn animation, this is called 'tweening. The animator can correct the result at any point, shifting keyframes back and forth to improve the timing and dynamics of a movement, or change an 'in between' into an additional keyframe to further refine the movement. One of the first applications of key frame animation was the award-winning 1974 animated short film, Hunger.
There is also an animation technique known as keyframing. Contrary to tweening, every frame of a keyframed computer animation is directly modified or manipulated by the creator, such that no tweening has actually occurred. This method is similar to the drawing of traditional animation, and is chosen by artists who wish to have complete control over the animation.
Consider the following examples, created using animated GIFs, wherein a key frame defines the starting and ending points of any smooth transition:
As applied to motion 
An object will move from the top left corner of the frame to the bottom right corner. One of the two keyframes present in this animation will contain the object at the top left corner of the frame, while the other keyframe will show the object at the bottom right corner. Everything in between can be interpolated smoothly.
As applied to shape transformations 
In a shape transformation, the first keyframe contains the original shape, while the ending keyframe contains the transformed shape. Shape transformations defined by keyframes cannot accurately show how the shape will be transformed in between the two keyframes. In Adobe Flash, shape hints can be added to both original and transformed shapes to give the program a better idea of the interpolation.
As applied to color transformations 
When an object changes its color smoothly, the animation can be defined by keyframes—the first showing its original color, and the second showing its final color.
Use of keyframes as a means to change parameters 
In software packages that support animation, especially 3D graphics packages, there are many parameters that can be changed for any one object. One example of such an object is a light. (In 3D graphics, lights function similarly to real-world lights: They cause illumination, cast shadows, and create specular highlights.) Lights have many parameters including light intensity, beam size, light color, and the texture cast by the light. Supposing that an animator wants the beam size of the light to change smoothly from one value to another within a predefined period of time, that could be achieved by using keyframes. At the start of the animation, a beam size value is set. Another value is set for the end of the animation. Thus, the software program automatically interpolates the two values, creating a smooth transition.
Video editing 
In non-linear digital video editing as well as in video compositing software, a key frame is a frame used to indicate the beginning or end of a change made to the signal. For example, a key frame could be set to indicate the point at which audio will have faded up or down to a certain level.
Video compression 
In video compression, a keyframe, also known as an Intra Frame, is a frame in which a complete image is stored in the data stream. In video compression, only changes that occur from one frame to the next are stored in the data stream, in order to greatly reduce the amount of information that must be stored. This technique capitalizes on the fact that most video sources (such as a typical movie) have only small changes in the image from one frame to the next. Whenever a drastic change to the image occurs, such as when switching from one camera shot to another, or at a scene change, a keyframe must be created. The entire image for the frame must be output when the visual difference between the two frames is so great that representing the new image incrementally from the previous frame would be more complex and would require even more bits than reproducing the whole image.
Because video compression only stores incremental changes between frames (except for keyframes), it is not possible to fast forward or rewind to any arbitrary spot in the video stream. That is because the data for a given frame only represents how that frame was different from the preceding frame. For that reason it is beneficial to include keyframes at arbitrary intervals while encoding video. For example, a keyframe may be output once for each 10 seconds of video, even though the video image does not change enough visually to warrant the automatic creation of the keyframe. That would allow seeking within the video stream at a minimum of 10 second intervals. The down side is that the resulting video stream will be larger in size because many keyframes were added when they were not necessary for the visual representation of the frame. This drawback, however, does not produce significant loss of compression when the bitrate is already set at a high value for better quality (as in the DVD mpeg2 format).
See also 
- Motion Blur, Key Frames and the Fencepost Error Frames are not points in time. Frames are periods of time.
-  How to use Keyframes in Sony Vegas.
-  How to detect scene changes using key frames.