Dimensions are of 3 types mainly in case of slowly changing dimensions:

Type 1 ->Doesnt maintain any history and is update only.

Type 2-> Maintains full history.Insert row when certain attributes change else update row.The latest record is identified by a flag or date field which is called the MRI or Most Recent Indicator

Type 3->Maintains partial history.History is maintained as columns and not rows.Hence it is predecided as to till how many levels of history you want to maintain.