history

It is common practice to simply type in commands into a terminal and leave the terminal with never clearing the history. The problem is, if someone accesses your system, they can see everything you have done, which can be an issue.

To check your terminal history, Open a Terminal and type:
history

To clear the History, simply type:
history -c

Now you can check your history again by typing:
history

And you can see it is clear. Do this before you log off your computer every time for good measure