Reply allows you to return a tell to whoever told you something last. When you are told something by anyone, it saves who told you in your body, and, if you reply with a message, it goes to them. You don't have to type in a player's name.
reply Thanks for that tell, Gunthar. This is a reply.
reply -a [msg] is used for replying to tells for more than one person. Only "reply -a" sends the reply to everyone who got the previous message as well as the original teller, e.g.:
One tells you, Two, Three, Four: Later. reply -a see ya You tell One, Two, Three, Four: see ya