What is the difference between send reply and forward?

The difference is that "forward" means that you would like to send that e-mail to someone who wasn't on the send list. "Reply" is when you specifically want to answer the person who just e-mailed you, i.e. I am responding to your question specifically therefore I am replying to it. And finally "send" is when you are finished writing your message and you want to now send it off. So the general order is you either forward or reply, type your message, and then send.