Lucid dreaming and Astral travel are different though related. They are similar in that you are experiencing a world different than the one your physical body is in. Generally a dream world is entirely constructed by you and thus you are potentially able to change whatever you want. Places you can go to through Astral Travel exist without your contribution. I will say that through advanced techniques, the two can become very similar. Some feel that the distinction between entering a world imagined by you and one created by others or some other means does not mean the actual traveling part is any different.
There are techniques to enter Astral Travel from lucid dreaming as some find it an easier transition than doing Astral Travel from being awake.