if you have an android you won't know the difference because iPhone sends sms to android phones. But if you sending texts between two iPhones Apple doesn't send sms..the text message bubble is blue. (Android is green text bubble) On iMessage you know when someone has read your text message: it says "Read 4:23pm" And when you are typing "..." Pops up kinda like on aim back in the day "so and so is typing"
When I turn off iMessage I send sms to iPhones too.
I'm not good at explaining shyt

hope someone can explain better
But everyone I text today getting annoyed lol
"Turn your iMessage back on!"