A User dimension is a dimension that is created by the end-user. The individual dimension members of a User dimension are defined from criteria from one or more standard dimensions.
Quite often, an organization wants to analyze their data by comparing one set of data to another set of data. E.g., data for a recent period compared to the same period one year earlier.
If the comparable data sets are as trivial as e.g. two years, then you would probably just a Time dimension on the Year level, but if the comparable data sets are slightly more advanced, then you would need a User dimension.
When you combine Dynamic Criteria with User dimensions, you will be able to produce dynamic analyses, such as: Compare revenue of Year to date to revenue of the same period last year.