WhatsApp is supported by iPhone, BlackBerry, Android, Windows Phone and Nokia. It is cross platform so anyone with those phones can message each other.
Turning on Save Incoming Media will automatically download all photos and videos that your friends send you through chats. This may incur additional data charges, so turn it off if you have a limited data plan. You can always download media individually that you want to save.
If you have a limited data plan, using WhatsApp may incur additional charges to your phone if you are not connected to Wi Fi. Shut off WhatsApp when you are connected to a 3G or 4G mobile network to avoid additional data charges.