Posters are single sheet printed pages used to convey messages or images in a large format. "Large" of course is subjective. Usually, however, a poster is any page printed larger than 16 by 20 inches. Sizes can go up to several feet on all sides and are limited only by the size of the printer or printing press.
Doing a poster with page layout software is a quick and easy way to collect and size images, add type up to several inches high and create colorful backgrounds and accents. Elaborate professional programs such as QuarkXpress, InDesign and CorelDraw are typical examples of this type of software. Lower end page layout programs such as Microsoft Publisher can also do poster work but may have limitations when it comes to maximum page size and color manipulation (A poster's size will correspond to whatever "page size" is specified in the program).
Illustration software such as Illustrator can also be used to create original and exciting posters. Using the postscript drawing computer language, these programs can generate just about any image the designer imagines. Although a little more complicated to manipulate than page layout programs, illustration software can generate layouts of every size with accurate color correction and image reproduction.
If you have Photoshop software you can use it to generate a poster. Just used the "canvas" size to make a layout area the size of your desired poster layout. Working in Photoshop will allow you to quickly manipulate and edit various photo and graphic images. Photoshop will also allow you to add type in every color available on the color palette. Again, however, Photoshop is less intuitive than a page layout program and it's many tools and options will take a while to learn and master.
All the software imaginable, however, is not worth anything if you can't get it a printout of your design as a poster. All of the poster generating software described here can save files as PDF, EPS (encapsulated postscript) or proprietary formats. If using a commercial printer for producing your output, be sure to determine the preferred format the printer wants in order to generate the most accurate and matching print of your computer generated poster.