A frame is a still image. Video is made up of many frames per second to create the illusion of movement.
The number of frames to be displayed per unit of time is expressed in frames per second (fps) or Hertz (Hz). These are different terms for the same measurement.
Key frames (keyframes) and I-frames are the same.†
Digital video compression techniques involve compression of frames:
- A full 'Intra-coded picture'
- 'Predicted' from previous frame(s)'s picture
- 'Bi-predictive' or 'Bidirectional' calculated from surrounding frames's pictures. Can painlessly be skipped.
† This Quora answer says that although the terms are used interchangeably, a key frame is an I-frame with the property of being a point from which a decoder can start decoding