iPhone is a physical device running the iOS operating system while Android is an operating system that is run on multiple phones. The iPhone brand (its actually about 4 or 5 models of phone, I forget how many) is wholly owned by Apple while phones that run Android are made by a diverse group of companies like HTC, Samsung, and Motorola. The Android system runs applications run in Java.
It's quite different in terms of developming Android based apps and iPhone based apps. The market for Andoid is large as it uses in many lower segment of mobile devices and even in smartphones and tablets.
The iPhone runs the IOS, and Android runs the Android OS. IOS runs on apple devices while Android OS runs on Android phones and tablets. iPhone is made by Appl, while Android phones are made by many different companies like HTC Motorola, Samsung and Google. Both the IOS and Android run apps. Apps are not compatible with each other.
Android has a much bigger share of the market than apple, so if your developing, may be worth looking at first. otoh ios is more standard across phones. Android updates aren't all applied by all vendors.