The key to stamp collecting is the classification of examples into distinct groups, making it possible to compare slight variation and hence value in any given design or denomination. Aside from the obvious illustration on the face of a stamp, collectors, dealers and catalogue makers use a standard set of other defining characteristics to describe philatelic material, providing a useful and internationally recognised language for enthusiasts. Even within the scope of one design lifespan, there can be variation as new plates replace those worn out by the printing process. 

Just as coins are issued in a range of denominations, so are stamps and this face value  one of the principal defining characteristics used by  philatelists. Different values are often represented using the same design in differing colours. The Jamaican pair below make an interesing comparison. In this case the value difference reflects the change to decimal currency in 1969. 

The colour of a stamp is also used as an important tool. Sometimes a slight change in colour is made deliberately, but in other cases variations may correspond to the use of a different ink on a new print run of the stamp. Much rarer are the odd survivors of a true printing error, where one or more colours or parts of the design do not appear at all.

Stamps are meant to be used once and discarded. The postmark signifies the end of their usefulness for mailing. "Mint" is the term used to describe a stamp which has not been used and is in the condition it was when it left the post office. "Unused" is a term for a non-postmarked stamp which has had the backing gum removed. The term "mint" can also be abbreviated and modified by the use of additional information regarding that stamp's past history. One such example would be  "MLH" (mint, lightly hinged) reflecting that previous mounting had left only  minor hinge-mount impressions on the backing. The collection of the innumerable cancellation marks is a whole field in itself. Some stamps are overprinted with another mark e.g. First World War Stamps, to indicate additional levies or charges. These can also form the basis of an interesting collection. 

Watermarks are another important feature of philatelic interest. They have been used to identify paper securities e.g. banknotes for centuries and reduce the liklihood of forgery. The watermark is made by compressing a design into the paper during manufacture, using special rollers.  As one might imagine, watermarks show up in water, but stamp aficianados use other, chemical methods to avoid damaging stamps.

In the early days of the postal service, stamps were separated from the sheet at the post office counter with scissors, but this soon ceased when the perforating machines was invented. Classification is usually made by the number of holes within a 2cm group. The number of perforations is dependent on a number of factors including, the machine's punch wheel, the paper, and even the direction of the paper's grain.

Paper is surprisingly important and many varieties have been used to print stamps including some with a special chalky surface coating, designed for sharper printing. It also made reuse of the stamp more difficult as the design was destroyed when stamps were soaked off the envelope.

Stamps often have slightly different dimensions from one print run to the next. This difference only appears when the plate from which the stamps are printed is dimensionally different. In the case of these stamps, the plate was made up of identical blocks called "clichés", cast from a master die and each bearing one complete stamp on its face. One hundred castings could be assembled into a plate to print a 10x10 sheet of stamps.  The design of the stamp on the left is from a cliché which is 17¼-mm wide while the stamp on the left is 18-mm wide.