Persuasion and presenting convenient images in politics.
While it may seem that information that reaches you through the media is more or less objective, reality is that the information is selected by various people. If the information itself is true, that doesn't mean you get to see all relevant information. You usually don't get the whole story, if there is such a thing at all.
PR departments of the Government, businesses or other organisations, and campaigning politicians make sure they hand out information that is favorable to them. They will not volunteer negative information, or information that doesn't serve a particular purpose for them. This means that the information you receive is always limited and subjective.
This also goes for the topics the information is about. If a government, for instance, talks a lot about war, other topics are in effect avoided. A presidential candidate may attack other candidates, in order to hide his or her own shortcomings. The information that is communicated overshadows what they want to hide.
Similarly, particular actions may be taken while a big event is taking place elsewhere, and that other news is intended to overshadow the news about the actions. A government might drop bombs on another country during the opening ceremony of the Olympics, for example.
Presidential candidates have been taking words of opponents out of context, and giving it a negative spin, does distorting what really was said. Newspapers and websites devoted to checking the facts, like www.politifact.com and www.factcheck.org have exposed such lies.
In order to avoid talking about inconvenient subjects, politicians may not answer questions in interviews or political debates, and instead talk about subjects that are more convenient for them.
While in Western countries the word "propaganda" has negative conotations of manipulation of information by the government, it is not so that governments aren't manipulating information. The fashionable word for it is "spin," where the person in charge of it is called a "spin doctor."