An infographic is a collection of data, data visualizations, text, and images that distils an argument into its clearest, most quickly understandable format. It is meant to persuade intellectually as well as illicit an emotional reaction. The primary difference between a poster and an infographic is that a poster explains an argument, while an infographic is an argument.
Key to a good infographic: Do not sacrifice the information for the visual appeal, and do not sacrifice the visual appeal to fit as much information on as possible. This is a balancing act. Pare down what you want to present to the essentials to get your point across and still create an emotional connection with your audience.