can I build an app without an iphone or ipad?

You can build the app and use the emulators. However, obviously, you'll want actual devices at some point. However, yes, you can build and test with the emulator and no attached physical devices.

 

Here is how it was as of a year+ ago, I don't know if this explanation is 100% current: You can download the development environment and run it for free. You have to have an Intel-based mac to do this. The development environment contains an on-screen emulator of the actual device. In order to load apps on your device you have to pay a fee to Apple and become a registered developer, it was $100 back then, then you have the ability to load your apps directly onto a limited number of devices. As a registered developer, once you have finished testing an app you can submit it to the app store.