Adding to your collection
Looking at the catalogue
In StampData, a description of a stamp type is a collection
of stamp fields (properties) that are sufficient to identify the type
Descriptions are basically a computerized form of what you say in conversation when you want to talk about a stamp but can't use the catalog number. For instance, if you're a US collector, you probably know what "C3" means, but what is its number in the Stanley Gibbons catalog? Instead you could say "US 24c airmail stamp of 1918", and that's the description. The formal StampData way to write this would be "issuer=USA-function=airmail-1918-24c". The hyphens separate the fields in case some have embedded blanks, and we use field names in cases where ambiguity is possible.
While descriptions sometimes have to be longwinded, in many cases they are rather short. In our example for instance, "airmail" isn't necessary because the only 24c cent that the US issued in 1918 was the airmail stamp.
Descriptions for overprinted stamps
For an overprinted stamp, the description should identify the base stamp type uniquely. If it doesn't, things still work, but for instance properties like color and watermark won't get filled in, because they come from the base stamp.
In practice, the way to get to the unique identification is to start with some generic properties, such as issue date and denomination, click on "Check" to see how many matches are reported, then add or subtract properties until there is just one match.
Descriptions in image identification
Image descriptions need not include the issuer, which is recorded separately. Also, the description is only really needed to narrow down possible types for identification, and so need not be unique. For instance, the US has issued fewer than 20 24c stamps, and a description of just "24c" narrows down to where the airmail stamp is easily found.