Print versus screen

What is the difference between what you see on screen and in print? Basically: a huge one!
There are a few technical parameters you have to bear in mind because a poster is designed on the screen for printing.

Calibration: Not all reds are equal
All printers and monitors vary in terms of their calibration. It would take huge technical effort to calibrate the screen and printer identically. If you then change to a different printer or a different screen, you are back at square one. This means the result you get from the printer is never 100% the same as what you see on the screen.

		Light and pigments Some programmes can switch between colour spaces or save graphics in the different colour spaces. When printing a poster, the recommendation is to save the poster and its components in CMYK.
		On pixels and vectors There are two basic types of graphics (and of course hybrids): Pixels (left) are picture elements. The more of them per unit of area, the crisper the image. However, lots of pixels also mean large file volumes, which require a lot of computer memory to process! Vectors (right) are linear elements. They are always equally crisp and only take up a lot of memory in 3D.

Most programmes for creating posters work with pixels.

A matter of resolution
Resolution is the number of pixels per unit of area. The most common unit is dpi (dots per inch). Nowadays, good screens have a resolution of 150 dpi. Older or cheaper screens work with a resolution of 72 dpi. A printer starts at 300 dpi.
Basically, even the cheapest printer is capable of turning an image that is crisp on the screen into something pixelated beyond all recognition.