Spearman's Rank Correlation is a statistical test developed by Charles Spearman in 1904 to determine the strength of the relationship between two sets of ranked data. It calculates a value using the differences between ranks to indicate how well the relationship between the two variables can be described using a monotonic function. The formula uses n, the number of samples, and D, the difference between ranks, to calculate D^2 and determine the Spearman's Rank Correlation coefficient.