What is WhatsApp?
WhatsApp is a messaging app that lets users text, chat, and share media, including voice messages and video, with individuals or groups. WhatsApp relies on data to send messages, like iMessage or BBM, so it doesn’t cut into your monthly text allotment. Thus helping to give you more time to chat then to talk
What is iMessage?
iMessage is an instant messaging service developed by Apple Inc. Launched in 2011, it is supported by the Messages application in iOS 5 and later and OS X Mountain Lion and later. Users can easily distinguish messages sent via iMessage, as message replies appear in a blue-shaded bubble as opposed to the Messages app’s standard green-colored bubble. Users of iMessage-capable devices can send an unlimited number of messages for free via iMessage. In addition to sending text messages, iMessage also supports sending photos, videos, Web links, contacts and location information. iMessage can also be used to send group messages to multiple people.
But the main distinguishing factor is that Whatsapp is available for both iPhone and android, but imessage is only available for apple devices running IOS.