Lock your computer
When you log into your computer, you get access to some resources on the basis of who you are. If you leave your computer while you are logged in, others can use your computer in your name. This can lead to information that only you should be able to see and change can be viewed and edited by others - in your name! So it is you who have the problem if nasty e-mails are sent from your e-mail address or the confidential information that only you have access to, ends up in public domain.
Always lock your computer if you leave it.
You can lock your computer by simultaneously pressing Ctrl + Alt + Delete and select "Lock Computer", or by simultaneously pressing windows button + "L" on Windows computers.