The main difference between an iMessage and a text are the colors. iMessage itself is clever and can tell if the person you are sending too also has iMessage. When you go to type in a recipients name that you want to send a message to, a little blue chat bubble will show next to their name if they have iMessage turned on. Your Messaging app will default to iMessage if the other person has iMessage turned on as well. Another indication is that your chat box will say "iMessage" or "Text Message" before you start typing it. Text messages are green so the send button will show up as green as well as your messages in the conversation with that person. iMessages do the same, but only in blue. Why is this so important to know? Because people who don't want to pay for text messaging must make sure that they're sending an iMessage versus a text.
How To Tell The Difference Between Sending An iMessage And A Text