I've always wondered why candidates have to "approve this message"; I mean, if President Obama is on camera talking about himself, then can't I assume he approves the message? Why does he have to state that he approves it at the end?
There's certainly a law that governs what must be said at the end of a political advertisement, or else President Obama wouldn't say exactly the same thing as every other politician at the end of an ad, but there's also an element of persuasion at work here. By appearing on camera saying that he approves the content, the President is giving the ad credibility. It's about him, his work, and his beliefs, and by saying he has approved the ad, President Obama is saying, "You can trust this information about me."
This appeal to credibility is known as "ethos." Ethos is a method of persuasion in which the speaker or writer (the "rhetor") attempts to persuade the audience by demonstrating his own credibility or authority.
I think the best way to understand this kind of appeal to the credibility of the author is to look at the three most common ways a rhetor attempts to demonstrate authority on a topic.
By now, you've hopefully gotten an idea of what ethos is: an attempt to persuade by appealing to authority or credibility. You might be wondering, though, what ethos looks like in writing or in speaking. Here are a few examples: