1. Screen Resolution;
iPhones are available in different screen resolutions; It is in 320×480 and the iPhone 4 screen resolution is 640×960. Same in caes of iPad; Currently it is 1024×768 and iPad 2 likely to have 2048×1536 screen resolution; So we need to care about this while playing with layouts; Here is a code to determine screen size;
CGRect currentScreen = [[UIScreen mainScreen] bounds];
currentScreen.size.width will return the width of screen;
currentScreen.size.height will return height of screen;
Note: If you rotate your screen to landscape, then height will become width and vise versa;
2. Device Type;
Following piece of code will tell you whether your application is running on iPhone or iPad;
UIDevice* currentDevice = [UIDevice currentDevice];
if(currentDevice.userInterfaceIdiom == UIUserInterfaceIdiomPad) {
NSLog(@"oh its iPad");
}else{
NSLog(@"This is iPhone");
}