When the initial release of 2001 was recieved poorly by critics, Kubrick decided to trim the movie in 20 minutes, and add a few things, to make it less confusing and faster. Very few people watched the movie on its original form, and there is absolutely NO existing copies of those trims today. Here is what he has taken out (as far as I can remember reading about):
* More scenes from Dawn of Men. The apes slowly get into that lake for the second time. On the current edit, the apes show up at the middle of the atack.
* More scenes on board of Discovery, showing Dave and Frank's life on the ship, including Dave playing on a blue-keyed piano. This cut is noticeable on the jump by the soundtrack (cuts exactly when Hal is shown).
* Frank preparing for his space walk.
* More scenes on the "bedroom". Dave walks around a bit more on the room while in his spacesuit, looking around. He picks up some food bars, and later notices a slipper at the bed (this is barely visible on the current edit). Then he moves towards the bathroom. If you watch the current edit, you'll see how he moves AWAY from the bathroom, and then, a moment later, he is walking towards it.
Here is what he added:
* Titles for Discovery 18 months later, and Jupter and beoynd the infinite.
* A small cut showing the black monolith when the ape is holding the bone, to show up the connection between the two.
Keep in mind that Kubrick shot TWO HOURS of film for every MINUTE shown on screen today, so there is a lot of material who didn't end up at the movie at all (Dave playing a psychedelic board game with Hal, Dr. Floyd doing a tour at the moon base, black and white interviews with scientists in today's New York City, the aliens, and so on)