If you have an iPhone what does this mean?

Ok so my new iPhone 5c is a week old & I'm getting used to it but I have a question. I get that when you text somebody who doesn't have an iPhone they get text messages, but if you text somebody with an iPhone they get an iMessage. But sometimes I will text friends with an iPhone & sometimes it'll go to text messages, not iMessage. Like I texted my friend right now & we were just texting, but then after a while we stopped texting & I asked a question, but that time it went to text message not iMessage & it did that before to like other friends that have an iPhone… What does it mean? Maybe they're phone died or was turned off, or that the message didn't send or what? In confused, please help!

All iPhones do this so nothings wrong because mine did it once and it just says it turns on and off sometimes

Sometimes the iMessage would not send in which case it would revert to sending a text. That's what happened.