The history shows log entries created by the system, by agent and by user.
Click on a log entry to open it. Additionally, you can click the expansion icon to bring up full history in a separate window, focusing on the history listing you've selected:
You can also use the expansion icon that shows the full history of the case:
When you open a history log that is an email, you can use the answer icon 
to reply to this email:
Overview
Content Tools
