What this difference between iMessage and text message?

I just got my iPhone not to long ago. I notice when I text some people it says iMessage does that mean they have an iPhone too or what?

I believe imessage is when it is sent to a persons email I think? I text my friend using it and it pops up as I sent it to his email?

IMessage is Apples own messaging ecosystem. When you text another iPhone or ipod touch or even an iPad (you can do that with iMessage) it will turn blue. When you text someone without an iPhone or another Apple product it will turn green. (Normal text message)

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