In a general sense, variance analysis is an “A vs B” comparison.  General examples of “A” and “B” are:
Current state of a model element vs a previous state of that same model element
Current state of a decision unit vs a previous state of that same decision unit
One plan scenario vs another plan scenario
One sensitivity vs another sensitivity
One version of a plan vs another version of a plan
Some specific examples that are common in planning are:
Budget vs Actual: This is a common form of variance analysis.  It analyzes the accuracy of my Budget projections. Did I achieve the aggregate plan that I set out to achieve?  Are there actions that I need to take to close any shortfalls vs Budget?
Scenario Analysis:  Best practice Business Planning includes the generation of multiple scenarios.  Comparison of potential scenarios is a key form of variance analysis.