There are 80,000 iPad apps. There are at least four times that many apps altogether. While iPhone apps will work on the iPad, and even say so under the compatibility list, they are half sized, or using the 2X button full-sized but fuzzy.
You have to pay attention to what you are buying. If you look at the screen shots, it's pretty clear which apps are designed for the iPad. An app with a plus mark in the price tag is universal, and will adjust to use on either the iPad or iPhone. Those are great bargains if you own both devices.