themselves

You use themselves when the people doing an action and the people receiving an action are the same people, but they are not you or the person you're talking to.

Example: They've hurt themselves.

Use for emphasis, when talking about more than one person or about one person when you don't know who that person is.

Example: The classes are as different as the students themselves.

If somebody or some people do something by themselves, they do it alone.

Example: The children earned $500 by themselves. I didn't help.

If somebody or some people don't feel themselves, they are different from usual, because they are sick, scared, stressed, etc.