As a collector of stamps it pays to be vigilant and scrutinise before deciding that you already have a particular issue in your collection. At a quick glance 2 stamps may appear to be identical but are in fact very different. This may be a watermark variation, perforation variants, subtle design changes or differences in the printing. Variances can arise from the use of a different printer or printing method and some occur regularly on each sheet of stamps due to the way the plates are prepared. Some collectors choose to collect retouches to the printing dies - these are areas of engraving that are re-cut as the dies wear during use - you will need good eyes to take that road!
Major varieties are usually listed in specialist catalogues. - Here are some examples as a guide.
1995 New Zealand 40c self adhesive. (images show postally used examples)
Note (1) the left downstroke on the letter A in the design.
(2) the arrow position extending into the letter B.
(3) the 40c value - less obvious is new zealand being a slightly smaller font on the right hand example
(4) and even less obvious is a "perforation" change with the vertical "perforations" having a different guage
(5) slight shading differnces
(6) different shapes to die cut corners
1982 Australia 60c Humpback Whale
The version at left is an extremely scarce Essay / Trial printing that was never intended for issue whilst the one at right is the common garden variety, The difference is that the Trial printing has a solid blue - green background whilst the regular version shows white streaking representative of sunlight shining thru the water which is also on the 3 other denominations in the series. It is belived that a small quantity of these trial sheets were inadvertently mixed with the final design sheets and issued for sale. A prominent dealer had aquired some of the trials at less than face value and used some as postage before discovering the difference. This shows that even experts can slip up as mint unhinged examples can change hands for many hundreds of dollars.