When creating a message on an iPhone, what does the blue chat bubble thing mean?

Whenever I'm sending a message, like creating one, and I type in a person, sometimes a blue chat bubble will come up. What does that mean?

If it's blue then the other person has an iPhone 4, 4s or 5. If it's green, the person has an iphone 3g or 3gs or has a different phone all together (samsung, htc, blackberry, etc).

It's iMessage, it's like BBM but between iPhones, iPads and iPod Touch

The phone is another apple product older than 3G. This is iMessage the free messaging service that lets iPod users text you without spamming you email!