Keyframes are used to create actions in an animation sequence. Keyframes are the main idea for animation in animation softwares, 2D and 3D. Keyframes are used to define the starting and ending points of the sequences. Keyframes are used to smoothen the transitions of animations, from breaking down into individual frames. 

Depending if it is 2D, where you have to make each keyframe and draw each frame, whereas in 3D, digital softwares can fill inside the starting and ending point of the keyframes. Main characteristics of using keyframes are, the importance of the individual frame from the shots, the keyframes that exist in animated and/or in live action scenes, and lastly the start, stop and end points for transitions.