A word class which contains words that add extra detail about the way an action occurred (i.e. the verb) but which can also modify another adverb or an adjective, e.g. 'The girl worked especially hard.' 'He was just too much!' Adverbs can give detail concerning time (soon), place (there) and manner (nearly).
An Adverbial is a phrase that does the job of an adverb provides extra information - usually about time, place or manner - in a clause. A sentence or clause can contain several adverbials (which, unusually for an English syntax, can be located in various places). Adverbials are usually 'optional' elements in a clause - its central meaning being reasonably unaffected if they are left out.
Twice a day ADVERBIAL time during the Summer ADVERBIAL time I exercise SUBJECT+VERB furiouslyADVERBIAL manner in the gym ADVERBIAL place on the rowing machineADVERBIAL place
An adverbial may consist of
a single adverb (I will do it soon) or
a prepositional phrase (I will do it in a minute) or
a subordinate clause (I will do it when I find my screwdriver)