Instant Messaging is to use an app to send messages to another device quickly, and easily. There are many IM protocols including iMessage, RCS, and SMS which are likely to be on every iPhone, and almost all Android devices. There is also IM clients such as Signal or WhatsApp, however the person you wish to communicate with must support the IM service you are using. iMessage will send using the Messages app to another Apple device. RCS will work with either Android, or Apple phone. RCS will use data, and therefore easier to communicate with someone, especially in a different country.