iMessage is a default messaging service on top of SMS on the iPhone where your messages to a sender appear in blue bubbles as long as the other person has an iPhone (yes, even if you don’t sign into your iCloud). If you message a sender who doesn’t have one, DISASTER occurs--your messages are now in green and you think of the counterpart differently. This stark call out of bad behavior in iMessage is probably one of the one biggest reasons of why people don’t move off of an iPhone. It forces you to think about the pain that the other person might feel when their message goes out in a green bubble should you make the switch away from an iPhone; making iMessage one of the biggest moats that the device has. The blue bubbles of iMessage helps justify owning an iPhone as it’s probably your most used app.