I don't understand iMessage?

What's the difference between iMessage, SMS, and MMS? Okay if I'm texting a iphone user and I use iMessage and they can't use iMessage will they still get the text? And if I can't use iMessage and they are using it and they sent me a text will I get it? Or no? Because I was texting this girl and I sent her a iMessage and then I turned off my data and then I turned it back on in about 4 hours and immediately I got a reply back from her. And when I say turn off iMessage will anybody get my texts? Will somebody explain all this to me?

IMessage is for sending text and images over the internet.
SMS is going through cellular connection; you don't need internet
MMS required cellular internet connection; can send images.

They won't get the text when they're not using iMessage. Nor reversed.
Turning off iMessage completely disables sending and recieving.

Look at this way, whether you have imessage on or off won't matter whether you receive or send SMS messages, you receive them either way. The difference is if you have imessage on then its always free text messages that don't count against your plan. It can save you money because you can use a smaller text plan if most of what you send are imessages. Even if you have unlimited texting you should turn it on as others may not have unlimited, so they can text you free. There are alot of advantages to that as you can even send imessages internationally free, and you can send them over cell tower or Wi-Fi, so even if you have an ipad or ipod touch you can send imessages free even without any type of monthly plan (over Wi-Fi).