Differences between Ipad, Iphone and ipod

What is the difference between an ipod and an iphone? I think the main difference is that you can make and receive phone calls with the iphone and you can´t with the ipod. I think both have access to the app store, but correct me if I am wrong.

If i am correct on that, then the iPad is nothing more than an ipod with a bigger screen, or an iphone without the phone capabilities and again bigger screen. Again, correct me if I am wrong.

Also, if I already have an iphone, is it worth to also have an Ipod and/or Ipad? What extras do I get from them?

Thanks