Propaganda is a form of communication that consists of influencing an audience toward a certain opinion or bias. This can be as subtle as saying, "nine doctors out of ten prefer" a brand of toothpaste or as overt as the anti-Communist "better dead than red" message of the Cold War-era. Propaganda can be used for political purposes and can also be used to influence an audience to purchase certain products or support causes.
Students can be taught about the styles of propaganda, each still common in political discourse and advertising today. You can discuss, for example, how "ad hominem" attacks (attacks on a person rather than an idea) are used in political campaigns or how half-truths are introduced to debates to distract from real issues. By educating students about the types of propaganda, they will have a better grasp of when someone is trying to use these techniques on them.
A good exercise to help students notice propaganda is to have them bring in a magazine from home. Have each of them pick out an advertisement from that magazine and describe what they think the advertiser is conveying. Ask if that message fits into any of the categories of propaganda. By noticing persuasive language and techniques, the students will be able to see those methods influence behavior.
Recognizing propaganda techniques and persuasive methods that rely on emotion rather than logic can also help to hone students' critical thinking skills. By using the same skills to approach other texts as they use to approach propaganda, the students learn how to use reason and logic to read critically.