adverbs 和adverbial 有何分別??

• "Adverbs and adverbials are similar but not the same. Though they share the same modifying function, their characters are different. An adverbial is asentence element or functional category. It is a part of a sentence that performs a certain function. An adverb, on the other hand, is a type of word or part of speech. We can say that an adverb may serve as an adverbial, but an adverbial is not necessarily an adverb."
  (M. Strumpf and A. Douglas, The Grammar Bible. Owl, 2004)

http://grammar.about.com/od/ab/g/adverbialterm.htm