About Cascading Styles
The idea of a cascading style is to base new styles on one which already exists. This form of parent/child style structure allows changes to be made to several areas of the document at once by simply changing the parent style.
For example, once a document is built, it may be determined that an incorrect font was used. By changing the font in the parent style, the change will cascade down to the styles below and update the various regions of the document which make use of those styles.
When creating a new style (On the Home tab, in the Styles group, click 
and then click New) simply select the parent style from the Based on drop-down list. To create non-cascading style, this pulldown option would be set to (nothing).
