The differences between the iPad and iPhone are huge, the size alone makes this device a must have for every man woman and child or a similar product. Once you start playing on the iPad going to a iPod or iPhone just seem weird, they are so tiny after the iPad.
What you can do with the iPad? I have pretty much replaced my laptop with this, I do remote desktop administration, I read books, watch streamed movies and tv, I teach my kids how to read and write, and draw, it has a thousand uses, and very few limitations. As a genuine item, you can do a lot, if you jailbreak it, well what you can do is unlimited.
Remember google is your friend and ifmyou can imagine doing something, chances are you can.