The email tab inside of a Party or a Case shows all email specific to that context. In a Party detail page, it shows all email conversations between the Party's primary email address and your organization. In a Case detail page, it shows only emails specific to that Case. 

In the above example, the tab "Emails (1/13)" means there are 13 total email messages and 1 of them is unread. 

ReplyOpens a window to reply to the original sender.
Reply AllOpens a window to reply to the original sender and all other recipients on the To and CC lines.
ForwardOpen a window to forward the email and attachments to someone.
DocumentsShows a list of all attachments that came with that email.
Send EmailOpen a window to send a new message.
Mark as unreadMarks the message as unread.
Mark as readMarks the message as read.