This is probably an embarrassingly noobish question.

I sent a standard SMS text message from my iPhone 6 to a colleague's iPhone, who mentioned that my message was green and not blue, and that meant I hadn't activated something or wasn't using the right app to take advantage of iMessaging.

Currently I don't use apps beyond what came with my phone. To send SMS text messages I use the green callout balloon button that's near the phone button.

My colleague said I could save money by using iMessage instead of paying for each SMS text message. I countered by saying that if it's not SMS then that means I'd have to keep my phone connected to the internet 24/7 to receive messages and I don't do that.

My colleague then said something like it does but doesn't use SMS, so I wouldn't have to maintain an internet connection 24/7 to receive prompt iMessages and yet would not have to pay for SMS messages when I did.

A quick search turns up plenty of tutorials about how to use iMessages but so far I have not found a clear and concise explanation.

Is it possible to explain in a few clear sentences how the messaging functionality works on my iPhone6 and when internet is used and when traditional SMS messages are used?