What's the difference between iMessage and text on iphone?

I checked my balance with optus and it says i have $0.00 but yet i can still text my friend. It says "iMessage" and when one message didn't send it gave me two option:
1. Try Again
2. Send with text instead

Does this mean that iMessage and Text is different?

if so, what's the difference? If not, why am i sending text with no credit?

Imessage is texting through the internet, not normal texting with your texting plan.

IMessage is texting only to other iPhones, iPads, iPod touches, and Macs running Mountain Lion

IMessage is for sending text messages between iPhone users. It's free for iPhone users. Text is your typical SMS and if you send or receive to a non iphone user normal carrier charges would apply.

Imessage is the app or software installed on your iphone… Just like or texting app that on itunes… This does not cost your texting plan but it will charge to your data plan… I guess

Text is like if you don't use the app… It will cost your texting plan per message…

Yes, iMessage & texts are similar but still different.

iMessages are exclusive to Apple devices & require internet access. (Free if sent over Wi-Fi).
Texts are universal, don't require internet access. (Also have a cost)