Dimensions are view-specific elements that show sizes and distances in a project. There are 2 types of dimensions: temporary and permanent.
Revit places temporary dimensions as you place components. You create permanent dimensions to define a particular size or distance. Each dimension type can be modified, and the component that is dimensioned updates accordingly.
Temporary and permanent dimensions, by default, use the unit settings specified for the project. For permanent dimensions, you can create custom dimension types that override the default unit settings.
Autodesk Video: 2013 Dimension Enhancements
Community Video: Dimensioning Tools