Propaganda is a marketing/communication mechanism which consists on using subjective information, or acts presented in a selective manner, with the aim of convincing and of directing the behaviour of an audience towards the desired objectives. The term is related to a manipulative approach.
For instance, the photo attached shows a poster from the WWI period, in which the enemy is represented as a beast that needs to be beaten. It tries to convince the public opinion about the necessity of the US getting involved in the conflict, even tough it was fought in Europe. This is clear when reading the sentence on the right: "If this war is not fought to a finish in Europe, it will be on the soil of the United States".