The stamp catalogue section is divided into years based on the date of issue.
There is an index page linked to keywords for each stamp.
Stamps are split into four types; Commons, Sports, and Variants.
Commons as their name suggests are the bulk of the stamps issued.
Sports are stamps which have an intentional difference. Some appear on the sheets but others can only be obtained from a Little Brown Envelope.
Variants are stamps with intentional differences other than the sport. The Llamedos stamps are a good example.
Imperfections are stamps with unintentional differences such as misaligned printing, major colour variations and smudges.
Overprints are existing stamps which have been overprinted to change the value or usage.