Set Analysis or better Set Expressions is an extension to the expression syntax that can be used within aggregation functions to further select sets of rows and sets of elements that should be included or excluded before the aggregation should be performed. Furthermore the Set Expressions also allows the use of set operators both between record sets and element sets.
Set Analysis is what one calls the QlikView Analysis when you have used one or more set expressions. It seems for some strange reason that the term Set Analysis rather than Set Expression is what is used most often. I find this a bit odd.
The best definition is what you find if you hit F1 and look up Set Analysis:
Aggregation functions by default aggregate over the current selection of field values. The current selection can be referred to as a set of field values. You can define other sets of field values and use them in your charts instead of the current selection. For example, in a dashboard you may want to show market share a product across all regions irrespective of the current selections.
Defining a set of field values is referred to as defining a set expression and using set expressions to analyze data is referred to as set analysis.
