A "difference series" refers to a sequence of numbers where each number is obtained by subtracting the previous number from the next number. This helps identify the change or difference between consecutive values in the series. Full definition
In effect, a matrix of difference series is formed for a large fraction of all possible combinations of station series pairs in each localized region. (climateaudit.org)
First of all, there are a couple major differences the series has done with this game. (reviews2go.com)
They say: «The algorithm starts by forming a large number of pairwise difference series between serial monthly temperature values from a region. (realclimatescience.com)