"Forgery" Terminology

 A good place to start is with the proper philatelic terminology to describe "forgeries" and other types of similar stamps.
The words forgery, fake, counterfeit are often used interchangeably but each has a specific meaning that should be observed
The most used term is the word "FAKE" since it generally means it is not genuine but Philately required the term be expanded to properly describe the many varied situations.

Forgery: Any unauthorized reproduction of a stamp or cover to defraud collectors. Some early forgers noted that their works were not intended as "forgeries" but fillers or facsimiles. This group is also described as "Philatelic Forgeries"

Counterfeit: a stamp made with the intent of defrauding the government or post office.

Although often used in philately, the word refers more to fraudulent money both bills and coinage. A more acceptable term is "Postal Forgeries".  Depending on the country some like the Roman States only had 2 postal forgeries while Spain had well over 100. Nowadays it is a multi billion dollar problem worldwide and we will not be discussing these worthless items.

NOTE - Postal forgeries tend to be contemporaries and appear normally during the validity period of the stamps issued by the PO. They might of course end up in packets after this or if a large quantity needs to be disposed of. Philatelic forgeries especially higher value ones can appear many years after the originals were issued.

Fake: any genuine stamp or cover that has been altered generally to increase it's value. This may be adding fake surcharges, changing or removing the perforations or adding fake postmarks. A fake cancel on a forgery is termed as a full forgery. 

This might also be the category of stamps cut out from souvenir sheets to be sold as the expensive original. 

Reprint: 2 categories, authorized reprints by the post or government and unauthorized reprints when the plates are sold or acquired by a non sanctioned person. In my opinion, these are little more than forgeries. Generally all reprints have some or many differences with the originals.

Reissue : A stamp of an earlier design printed and officially issued later (usually long after the original printing) which may or may not be printed from the original plates or dies. When the original plates and dies no longer exist, a new die is prepared which may have slight differences from the original.

Bogus: any stamp that never existed including a non existent color fabrication of a genuine stamp. Stamps that were genuinely produced but never issued are not bogus. Phantoms or Fantasies are a close cousin of these and are produced by a fictitious postal authority. Many third world countries issue these. Some collectors might designate them as Cinderellas but when they are sold as "rare" varieties they become fraudulent.

Similitude  these were not produced with the intent to defraud. They should have some indication on them (generally the reverse) that they are Facsimiles. They might be commemoratives or used on commercial letterheads. There is always the potential for these to be used for forgeries.

Facsimile A reproduction of a genuine stamp generally for illustration purposes in catalogs, books, souvenir sheets. These, depending on coloring can become forgeries or be used by forgers as their samples since the catalog ones give their creations some credence. A facsimile should be labelled as such on the reverse or the front otherwise it falls into the category pf a forgery or counterfeit.

Reproductions: This is a very common modern enterprise usually of rare stamps sold as "fillers". The thought being that since they cannot afford them, collectors are buying them to fill album spaces. Some are listed as being "forgeries" with value which is not the case. On the other hand they are sold as being genuine so then a case for being forgeries might apply.

Remainders: are surplus stocks of legitimate postage stamps that are put on the philatelic market after ceasing to be valid for postal purposes. The large quantities of remaindered stamps to the public is that used stamps often results in genuinely used ones being more valuable than mint ones.

Proof: A trial printing of a stamp from the original plate or die. It might be different than the final product. Proofs are often not colored or show perforations so they are an easy target for forgers to duplicate. With modern technology it is simple to turn a genuine stamp remove the perfs, print it on heavy stock  and turn it into a "proof"

Trial colors: These can be legitimate non postal sought after creations made by the original designers or printing houses. However, with modern technology, a lot are "created" and sold as original trials to defraud collectors. This tends to be a growing problem.

Essay: A trial printing of a proposed design for a stamp. It may be very similar to an issued stamp or completely unlike anything ever issued. Here again we have an issue with forgeries and bogus essays that never existed.

Can you fake old paper to make forgeries? Early forgers would use printer's waste or margins (selvage) from period low value stamps to make forgeries. Also a simple method is old books. These books often would have many blank pages that could be converted. Some  like Sperati would chemically remove the image from low value sheets and replace it with a valuable image from the same period.

Why were very low value stamps forged?

I noted a comment about Postal Forgeries on a large stamp forum where a member mentioned an article from a prominent dealer/auction site that early postal forgeries were basically non existent and the bulk of forgeries were made for the packet trade or collectors. Other "expert" members agreed.

This is simply misleading as Postal & Philatelic forgeries are directed at specific markets.

The amount of Postal Forgeries in the early periods really varies by countries, one blog I have done on the Roman States shows only 2 rare and valuable postal forgeries while a country like Spain or Cuba was primarily all postal forgeries.

It makes sense that postal forgeries would be produced for actual use and to replace basic consumer stamps.