Fakes are generally common stamps altered so as to appear to be scarcer, more valuable ones, or stamps which have altered so as to give an "improved," false appearance as to their condition. The types of fakes most prevalent in the current philatelic market are fake overprints, reperforated stamps, stamps with perforations trimmed off so as to appear imperforate or as scarce coils, stamps with fake grills, and stamps which have been exposed to certain chemicals, so as to make them appear to be rare or missing color varieties. Common condition alterations include: bleaching cancellations off of used stamps to make them appear unused, filling thins or obscuring them with hinge remnants, the application of false postmarks (in cases where used stamps are more valuable than unused ones, or rare postmarks exist), and regumming.
Forgeries are labels created "from scratch" by the forger in order to resemble particular stamps. They come in three types: postal forgeries, created to defraud the postal service, governmental forgeries, created to deprive an enemy nation of funds from the sale of its stamps, reprints, and philatelic forgeries.
Postal and governmental forgeries need not concern us here, because they are generally not forgeries of scarce stamps. In fact, many postal and governmental forgeries are more valuable to collectors than the actual stamps that they resemble.
Reprints are produced, either governmentally or privately, with the printing plates that were used to produce the original stamps. Often, stamp catalogs note the differences between the original stamps and their reprints, when reprints exist. Many reprints were produced quite legitimately as novelty labels to be sold by packet makers, and major differences usually exist between originals and their reprints.
Philatelic forgeries range from the crudest, most obvious reproductions to some that are extremely convincing. In the 19th century, many considered it quite acceptable to fill a space in an album with a facsimile when the genuine stamp was unavailable. Some of the early forgers, such as Fournier and Sperati, were true masters of their craft, and their forgeries are valued by collectors.