In general, a reply is a verbal or written response to a proposed question. For example, responding to an e-mail is considered a reply.
The difference between "reply" and "reply all"
Replying to an e-mail is analogous to sending a letter back to someone, but over the Internet. When you have received a message, e-mail clients have the Reply or Reply all option. The difference between the two is that Reply only sends your response to a single individual. Reply all sends it to everyone who initially received your message, as well those who were CC'ed (carbon copied).