graphic design, illustration, photography
Posters are the epitome of graphic design. You have just one sheet of paper to show one idea.
The most important thing of a poster for any event is to communicate effectively the main point of the theme. Once you set up your idea and the compulsory titles and dates and sponsors it's time to play.
I use first my beloved blue mechanical pencil to sketch several ideas. Then I choose one and decide how I will be doing it. I could take a photograph or create a complete illustration using Photoshop, Illustrator or even Zbrush and Cinema4d. Finally I set up the complete poster with InDesign.