Why Are Some People's Text Bubbles Blue & Some Green on My iPhone?
What your text messages say obviously matters far more than what they look like. Still, there are things you can't help but notice, such as the fact that some texts you've sent have a green background and others have a blue one. The green background identifies normal text messages, while the blue background highlights texts sent through iMessage.
Texts sent with iMessage go through your iPhone's Internet connection rather than through your phone carrier. Because of this, they are free to send and receive, as long as you're on a Wi-Fi network or you're not already almost over your data transfer cap. On your iPhone, iMessage integrates seamlessly with the texting system: if a text can be sent through iMessage -- if the friend you're texting owns an iPhone, in other words -- it automatically will.
2 Other Apple Devices
You can also use iMessage on other iOS devices, such as an iPad or an iPod Touch, and on Macs running OS X Mountain Lion or newer. As long as you're signed in to the same iCloud account, your messages are synchronized across all your devices, meaning you can start texting with a friend on the bus and then pick the conversation up on your computer once you're home.