as yoink correctly said they both mean the same thing.
tweening is the process of generating intermediate frames between two images to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings between the key frames which help to create the illusion of motion
you also have shape tweening which is used for when you want and image to "turn" into another image, however it is not very commonly used as it distorts between the frames.
just in case you wanted to know the different of shape and motion