I'm typing this on my iPad.
There are a lot of similar apps. Some apps work on both iPhone and iPad others just iPad/iPhone. Good free apps (that I use) are we far, we rule, aim, and others. But the majority of the apps like with the iPhone are paid for, but are more expensive for the better ones.
Edit: if the app is made specifically for iPhone it gives you the option to scale it up to fit the screen. Other wise it's centered in the screen at iPhone screen size.
Edit 2: you have to download apps. Either via your computer with iTunes or on your iPad with the app store app. Your iPad already comes with preinstalled apps like the calendar, settings, notepad, contact book, iPod, videos, and photos(I might have forgotten a few).