People often ask what is meant by 'RP'.  RP means Real Photographic.  The difference between a Real Photographic postcard (RPPC) and a printed one, is easily seen under magnification - as I have illustrated below using New Brighton Lighthouse as the subject.  A small 10x magnifier (which folds up, and can be kept with you while visiting fairs, antique shops etc) can be bought from any good stamp accessories supplier or camera shop.  A Real Photo postcard can either have a glossy or matt surface.

 Real Photo      Printed

Chromo-Litho cards are neither of the above but are high quality cards printed by a method that layers down solid colours (unlike modern screen printing, which produces the effect of 'dots', above right).  Again, these can easily be distinguished under magnification, as seen below.  These are mostly pre-WWI and printed in 'Old Germany', e.g. Bavaria, Saxony, Prussia etc.

 Chromo-Litho

Did you know?
Lighter fuel (highly refined petrol) - removes finger prints and dirt from glossy RPs.
Clear Face Bags - a much cheaper way of protecting postcards than PVC pockets - from stamp accessory suppliers.
Fruit trays (free from supermarkets!) are made from very light, very strong, cardboard. Ideal for mailing postcards.
Stamps - a knowledge of issuing dates (along with postage rates) helps identify date of posting if the postmark is unclear.