I have GSM, but what is ROAMING and how does it work on Sprint?

I have GSM, but what is ROAMING and how does it work on Sprint?

I dont have an I.T. degree, so can someone please be detailed about what ROAMING is and be specific? How does it work? Do i turn it on in the phone settings? Do i get charged, etc?

Roaming is when you use another carrier's tower to receive cell service. Most cell phone companies don't charge for roaming anymore as long as you're in your home country (USA if you're from America) but you will be charged if you're from America and you go to England for instance. Check your bill statement to make sure that roaming is included with your phone plan.

On my phone, when I'm roaming, I see a small triangle.

Cold is right, roaming will show as a red triangle.

When i am in the USA i roam on ATT primarily and TMO if ATT is not available.

I am yet to roam in Canada