Is messaging other iPhones free?

Ok right now I'm a little worried because I don't have texting and I have a iPhone 4S. I just sent my friend some texts. He has a iPhone 4.Is texting other iPhone free like my sister said? And my messages are green to my friend not like when i text my sister her texts are blue when I text my sister she has the iPhone 5. Should I be worried? And will this effect my plan? And I was texting over Wi-Fi

Yeah if you are using iMessage this is just effectly email.

How are you texting over Wi-Fi texting doesn't require Wi-Fi or data and it depends what carrier you have if you have Verizon the only thing you get charged for is playing games, and the internet if your not on Wi-Fi if you have an smartphone then you do have texting and if your sister said it's free it probably is

Calling isn't but imessage is