A poster is a temporary promotion of an idea, product, or event put up in a public space for mass consumption. Typically, posters include both textual and graphic elements, although a poster may be either wholly graphical or wholly text. Posters are designed to be both eye-catching and informative. Posters are one of the most cost effective forms of advertising on the market. Posters allow you to spread your message to a wide audience far cheaper than radio, print or television advertising in a simple, easy and affordable way. 2. Posters are credible.