It saves after you exit your world automatically. No need to save by hand. You can have multiple worlds though. Heck you can even download other's map if you have Androzip app. Simply dowmload someone's file and unzip it and move their save to the games' main folder. You then open it up and play in their world with whatever they built on it and you can build upon it or destroy it. I just learned how to do this today and played a couple that two people uploaded on the minecraft forums. Real neat.
You can also create a zip file and share your creation through mediafire or other means of upload so others can play your map. Not that many people know this but little by little more people will know about it hopefully.