That really depends on the apps. Some apps are made for specific phones and are not compatible with other phones. So not all apps that work on iPhones will work on a, say, Android. A lot of apps do make compatible versions of a app for different phones though like eBuddy which works on iPhones, Android and my Nokia. It's a matter of researching the app you want to download.
Yeah. But you could get the iPhone apps from the Apple iOS app store, that are apps pitch to the iPhone.
And other phones might have specific app market, like Android phone, you can get android apps from Google play market.
BTW, this android apps list would interest you: http://funadvice.com/r/16a3l9os5al