What's the difference between iMessage and text message on an iPhone?

So I got an iPhone 4S for my upgrade, and when I text certain people who have iPhones it comes out as iMessage, in blue. And yes I text their number, not email. However my dad has an iPhone too, and when I text him it comes out as text message. So I'm kinda confused?

IMessage is from iPhone, iPad or iPod touch to each other and only works with each other. Texting is phone to phone and can't be used with iMessage.

His iMessage is probably turned off. Go to settings>Messages>then turn iMessage ON.

iMessage goes over Apple servers and does not count against your mobile carrier's data useage, while SMS goes over the cellular signal and does count against your monthly data count.

I'd always keep it on.