Advertising - A special group of cards used to advertise businesses or products. Many stores used postcards to advertise new items and use them like coupons. This Lipton's Cocoa came from a local grocer and offered the receiver a free 1/5 pound of this new product. 

Artist Signed - Usually these cards are reproductions of pictures, drawings or paintings signed by the artist who drew them. You will see many holiday and greeting types of these. When the cards are printed, the artist's signature is also reproduced on every card.

Arcade Card - Typically not considered a postcard. Cards are generally same size as postcards with a picture of some sort on one side and no printing on back. They differ from postcards in that there is no mailing information printed on them.

Continental Type Card - Generally a larger type postcard than the more standard 3 1/2 by 5 1/2 size. Continental is 4X6 or larger.

Hold to Light - These are a special type of postcard, generally scenes with holes in the card. The holes would either be all of the way through the card or perhaps the card would be made in layers with a very thin paper covering the holes. When the card is held up with a light source behind it, it appears as if where ever the holes are, that a light is on. (ie., a city street, or a starry sky would appear to light up.) Many of these are rare, as they were more expensive to print, and fewer made. Many were of Christmas Scenes. They can range from $30 and up.

Leather - Postcards printed on pieces of Leather. Typically burned on, some with few colors. Also, there were a few wooden cards made in a similar fashion.

Real Picture AKA RPPC, RP REAL PHOTOGRAPH - Usually very collectible as the older black and white photographs show great detail into the past. Many early photographers reproduced their work on postcards. Eastman Kodak introduced special paper in 1902 that made printing pictures much simpler. In 1903 Kodak even released a special camera just for taking postcard pictures. These types of cards can be distinguished from a lithograph or other printing type by using a magnifying glass to look at the detail in the picture. Cards other than real pictures will have a dotted or grainy appearance from the printing process.

VIA Wikipedia postcard

Sets and Series