Offset Printing is the most common form of printing used today. It is used to publish brochures, books, pamphlets, cards, flyers....you name it.
Printers can either print solid flat colors with Photographic Images in the form of Half Tone Dots or they can print Four Color Process (or even both together). When a printer uses solid flat colors this process is called Spot Color Printing. When a Printer prints Cyan, Magenta, Yellow and Black (CMYK) This is called Four Color Process Printing. This page will explain these processes in much more detail.

Below is a simple diagram showing how Offset printing works . The Ink adheres to the Printing plate and is passed onto the Blanket roller. From there it is transferred to the paper as it is fed through the printing machine.

The Brochure and CD below were printed in two colors - Black and Blue. Photographic images created using 2 colors are called Duotones

By separating artwork into four colors CMYK the printer can reproduce a wide range of photo realistic colors by arranging these colors in different densities and dots. The dot patterns printed are too small for the naked eye to see (usually 150 dots per inch or finer). Your eye actually does a lot of the color mixing.  
Below is a very close up picture of an area of a photograph printed by Offset Printing. You can see the dots and the dot patterns.

The dots in the CMYK separations are printed at different screen angles 45 degrees apart so as the dots don't overlap each other and cause unwanted effects.
When a printer gets his printing plates out of alignment you can get an effect called a Moire Pattern as in the Image below.

The Brochure below was printed using Four Color Process Printing. CMYK

There are many programs used to produce artwork that can be sent to a printer.
The most popular programs used by the established Graphic Design industry are Adobe In Design and Quark Express.
Corel Draw is also very capable of producing high quality finished artwork.
There is also a freeware program called Scribus which is capable of combining images and text to produce high quality CMYK artwork.
All of the above programs can export artwork in the PDF file format in CMYK color. PDF files embed all of the text fonts, images and graphic elements. They retain Vector elements in their original form as well as Bitmap elements. Exporting your artwork as a PDF file means that the printer does not need a copy of the program you used to create the artwork, nor does he need the fonts you used installed on his computer.
Please visit our Desktop Publishing Page for Tutorials and Lessons on the above programs.

If you are a beginner you must read the Essential Background Pages first.

The Do's and Dont's of preparing artwork for Offset Printing.

Go to the Desktop Publishing Page for more Information on each of the Programs used for preparing art for Offset Printing. Make sue you read the Tutorial on The Do's and Dont's of Offset printing.