Root beer vs Sarsaparilla
 

 Sarsaparilla is derived from the root of the Sassafras tree. It is a first cousin to root beer. I many ways it was the original 'root beer'.  Sarsaparilla is based on just one root, it is not a beer of roots.  Root beers are a mixture of many different types of roots, they are a "beer of roots". The type, quantity and blend used is quite often a very strongly guarded proprietary secret. Sarsaparilla will have a stronger root beer taste or aftertaste.  Root beer may be blended to the public taste and acceptance.  Sarsaparilla was at one time brewed from the bark of the Sassafras tree. This was declared a carcinogen, so now the bark of the root is used. 

Root beer can be derived from any number of roots.  Numerous combinations of roots and ingredients, such as: Dandelion, Sassafras, Anise, Birch, Cinnamon, Clove, Licorice, Vanilla, Wintergreen, and many other barks and herbs may be blended in different quantities to give root beers their different tastes.  

more Sarsaparilla on the Sassafras page

A wonderful article on the Colorful History of Sassafras

by Priscilla S. White,  Penn State Master Gardner
highly recommended

sarsaparilla (särs'pərĭl`ə, săs'–), common name for various plants belonging to two different classes and also for an extract from their roots, formerly much used in medicine and in beverages. True sarsaparilla is obtained from various tropical American species of the genus Smilax (which also includes the greenbrier) of the family Smilacaceae, sometimes joined in the Liliaceae (lily, common name for the Liliaceae, a plant family numbering several thousand species of as many as 300 genera, widely distributed over the earth and particularly abundant in warm temperate and tropical regions. Most species are perennial herbs characterized by bulbs (or other forms of enlarged underground stem) from which grow erect clusters of narrow, grasslike leaves or leafy stems. A few are woody and some are small trees.
Other plants used as substitutes for sarsaparilla include the wild sarsaparilla (Aralia nudicaulis, although S. glauca also bears that name) and the American spikenard (A. racemosa), both North American plants of the family Araliaceae (ginseng (jĭn`sĕng), common name for the Araliaceae, a family of tropical herbs, shrubs, and trees that are often prickly and sometimes grow as climbing forms.

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