I am very interested in developing iphone or ipad application. As far as I understand, I need a mac to develop iphone or ipad application.
I just wonder if I can develop ipad or iphone application on ipad with keyboard.
To develop an iOS application you need Xcode (the Apple IDE) and the iOS SDK for the latest version of iOS you intend to target with your app. Those can only be installed on a Mac running Mac OS (unless you count "Hackintoshes"). Get them both at Apple's developers' site.
So basically you need a text editor which uploads files to server (at minimum), Gusto is one such application.
There are also other differences between native and web applications.