Difference between [author] and [user] tokens?

I have noticed, when configuring an content link to send out an email, that both the [author:user-name] and [user:user-name] tokens give the same value: the username of the acting user. I thought that the [author:user-name] token was supposed to give the username of the node author. The [author:user-mail] token also gives the email address for the acting user. Is there any way to get the username or email of the node author?