Some conferences do position posters as an official participations in the conference with an article, while other as an un-official. The difference between those is – whether the article will still be included into the conference proceeding or not. If it will be included then it is treated as an article when the author has no right to talk loudly about it unless somebody asks. Otherwise it is a way for students to communicate the work and discuss, but it will not count for the PhD requirement to have x articles published, and so is not worth to accept the invitation in majour cases. Notice that sometimes those are printed in a local proceeding. Therefore you should be very careful if your work is accepted to the conference as a poster and find out in advance what it means from the publication point of view and then check with your supervisor whether it counts or not for your PhD articles requirement.
There are different ways to compile a poster. Mostly posters are prepared by authors in a classical way as a A1 or A2 size paper that contains a composition showing the logical flow of the paper from introduction to the conclusion. Some authors doesn’t bother to produce something creative picture and just put on the poster a set of slides they would show if they are allowed to present the work as a talk. Minor number of authors will just print out A4 pages and put them instead of A1 having printing out nothing more than the text of the article.
There are some examples from ICEIS’08 conference.