What is the difference between blue and green iMessage texts?

Yes, green means an SMS text instead of an iMessage (which is in Blue).

The BIG difference is that the SMS texts COST MONEY and apply against your text plan (think Green bubble = money!). The iMessages in the blue bubbles are free.

You can only send SMS texts (green) to phones that are NOT Apple devices (e.g., you text a friend that uses a Samsung device). Nearly every Apple device, including iPhones, iPads and Macs can use the free iMessage service, provided both parties are using the most up to date operating systems on their Apple devices.

Green (I think) indicates a standard SMS text (pretty much the type of message any cell phone sends and receives). Blue is a Messages text sent to another iOS device.

On the iPhone a green message is one sent by SMS, the traditional phone text messaging system, a blue one is via iMessage to another iDevice.

<Edited by Host>

Judging from the product list on the post, I think the OP has an iPhone.