Why do people with iPhones receive email text messages?

I have an iPhone and other people I text with iPhones tell why when I text them they get my email instead of my phone number. Why is that is it possible to not to ever send out my email just my number. When I text them with my iPhone.

In your settings for imessage you can set it up to use email or phone number as your identifier, thats why those with ipad and ipod touch can still send imessages without paying monthly plans, they just assign imessage that way.