The basic principle behind offset printing is that oil does not mix with water. Machine offset printing, also known as lithographic printing, uses metal plates that are made from films using a photochemical process. The plates are exposed to light through the films and then chemically treated so that non-image areas are water absorbent, and repel ink. For each color, a different plate is used. There are four different colored plates, known as the four-color, CMYK printing process. (Cyan, Magenta, Yellow and blacK.) This differs from RGB (Red Green and Black), known as the three-color printing process.
Offset printing is only offered by professional printers. There are printing companies that also offer graphic design services, and can make your entire invitation for you. Or, you can design the invitation yourself, using downloaded stock art or original artwork, and then bring your design to the professionals.
If you choose to do the latter, contact the printer first to ask how your files and images should be saved, and what kind of design software they support. Your images will most likely need to be saved as CMYK and some printers will require that you produce your invitations on a program such as Adobe Illustrator. In this case, you might need to enlist the help of a graphic designer who has this software.
Another benefit of using machine offset printing is the use of "bleeds." This means an image can run off the page, rather than being limited to the frame of the invitation. Perhaps you want an image of a flower to look as if it extends beyond the page, this is where a bleed comes in. It is hard to accomplish this on all but the most advanced home printers, unless you actually cut the paper or invitation after it is printed. To accomplish bleeds, familiarize yourself with the invitation-making program that you are using, there is likely a bleed setting you will need to use. You also may contact your printer ahead of time to ask if there are bleed margins you need to use.
Die-cutting is a technique that can be used together with offset printing. A die-cut machine is a device for scoring, cutting, stamping and embossing images. The technique can be used to cut your invitations into a specific shape, such as a heart or a bell. Or you can use die-cut embellishments (images precisely cut by using this technique) to add to the invitations once they've been printed. You can find die-cut embellishments at craft stores. If you're interested in an actual die-cut invitation, ask around for printers who offer this service.
The options for using machine offset printers are many, and each printing company offers a different variety of services. Ask for recommendations, and search around for the best prices. Often universities have a printing company, and are able to offer good prices.