Propaganda is a form of communication to distribute information. It is always biased. The information is designed to make people feel a certain way or to believe a certain thing. The information is often political. 

It is hard to tell whether the information is true or false. Very often, the information is confusing and unfair. Propaganda does tend to make disputes last longer, and be more difficult to resolve. It can take the form of posters, TV advertisements, and radio announcements. 

The word 'propaganda' comes from Latin. At first, it meant 'ideas to be spread around'. But in the First World War, it came to mean political ideas that are supposed to be misleading.

Propaganda is like advertising in some ways. For example, it uses the mass media to spread its ideas. But advertising is usually trying to sell something, whereas propaganda is about ideas. It is often political, and used by states or political parties, not private companies.

Uses 
Propaganda is often used during wars.  There it can be very useful.  Sometimes it keeps the people of a country happy  telling them that their country is fighting well and telling them how important it is that the enemy is defeated. Sometimes it tries to make people hate the enemy. The information could tell people that the enemy is evil or make them seem not human. Sometimes a government gives propaganda to the enemy  telling them that the war is going badly for them and that they should stop fighting.

When a country is not at war, propaganda can still be used. The government may use propaganda to change what people think about a political situation. A group may try to change the way people act towards an issue.

Propaganda under some countries, like dictatorships, is used along with censorship. While propaganda tries to give people false ideas, censorship forces the ones who disagree with propaganda to keep quiet. Then the propaganda can say everything, because nobody can question it in public.

Propaganda is also used to win people by tricking them. Some people say that cults use propaganda to get people to join them.

Examples of propaganda:
 British propaganda against Germany in the First World War.
 German propaganda against Poland to start the Second World War, see Attack on Sender Gleiwitz
 Propaganda through mass media (print, radio and movies) was used by the Soviet Union from beginning to end. Some great artists, like Sergei Eisenstein and El Lissitzky, helped them do it. Many others, such as Solzhenitsyn, did not.

History 
Propaganda has been used in every known civilisation. It was used by Rameses II on his monuments in Ancient Egypt; it was used by Ancient Greek orators; it was used by Julius Caesar, and all Roman Emperors. The word itself is formed from propagate, meaning to multiply.

Propaganda was carried much further  by the Sacra Congregatio de Propaganda Fide of the Catholic Church. This committee, founded in 1622 by Pope Gregory XV, had action branches in most European countries. These were the local branches of the Inquisition, which sought out heretics. With torture and the threat of death by burning at the stake, they forced heretics to recant (to publicly withdraw their previous beliefs). The objective was to remove all challenges to the supremacy of the Church in matters of belief. The 1578 handbook for inquisitors noted  "Punishment does not take place primarily and for the correction and good of the person punished, but for the public good in order that others may become terrified and weaned away from the evils they would commit".

References 

Politics
Communication
Psychology