Short answers, pulled from the story.
A key frame in animation is a drawing or shot that defines the starting and ending points of a smooth transition. The position of key frames on a film strip or digital timeline determines the timing of the movement, and the frames in between, called inbetweens, fill in the remaining motion.
Inbetweens are the frames that fill the gaps between key frames. Because only two or three key frames over a single second of footage cannot create the illusion of movement on their own, inbetween frames are required to complete the motion.
In 3D graphics software, key frames can be applied to nearly any parameter of any object, not just position. An animator can set a starting value and an ending value for a parameter such as a light's beam size, and the software automatically interpolates between them to create a smooth transition.
In video compression, a key frame, also called an intra-frame, is a frame that stores a complete image in the data stream. All other frames store only the changes from the preceding frame; a key frame is required whenever the visual difference between two frames is too large to encode incrementally.
Key frames are inserted at regular intervals to allow seeking within a video stream. Because non-key frames only record differences from the preceding frame, a viewer cannot jump to an arbitrary point without a nearby complete-image key frame. Inserting one every ten seconds, for example, allows seeking at minimum ten-second intervals.
In non-linear digital video editing and compositing software, a key frame marks the beginning or end of a change to a parameter such as audio level. For instance, a key frame can be set to indicate the exact point at which audio has faded to a certain level.