Connect to iOS iMessage Messages Chat iMessage uses an internal messaging system (similar to WhatsApp) that is independent from SMS so you can chat from iOS to iOS without SMS and without having a SIM card inserted.

Is it possible to chat from your Android device with someone on an iPhone that only uses the iOS iMessage Chat client?
iMessage automatically uses SMS messages for contacts who don't have an iPhone, so the answer is yes: you chat with them by sending an SMS message.

Apart from using SMS, there's no way for non-Apple users to get messages into iMessage. The protocol is closed and Apple does not have clients for other platforms, nor an interface for other apps to communicate with it. They designed iMessage not as a chat system in itself, but as a way to quietly redirect iPhone-to-iPhone SMS messages to use their servers instead of the SMS network.